I think referring to her by first name is best. If she doesn't deserve the title of mom based off behavior, don't give it to her

Sometimes people who have a really shitty relationship with their parents don't refer to them as their parents. If that's the source of this I wouldn't feel so bad, especially because it seems like this issue kind of beats you up.

If the reason you dont feel like calling her 'mom' is that she wasnt a great parent(as opposed to her not being a "feminine" enough parent) then I wouldnt call it bigotry, just your detachment from her.

I don't think you owe her calling her "mom" inherently just to respect her identity, but if your reason is "she was never a mother figure to me" that is something you might want to reflect on, as it's a bit vague.

Much like SmokinStalin said, if that means "she wasn't a good parent", you're fine, if that means "she wasn't stereotypically feminine or didn't fufill the like, emotional labor role of mother" pre-transition, idk that could have some brainwormed roots worth teasing out and eliminating. You could also just decide its easier to still call her mom. Plenty of people have shitty mothers and will understand that mom =/= good mom

It depends how your parent wants to be referred to. In abstract though I would say that "mom" is better if she identifies as a woman. If the situation was revered and a parent referred to their child as "son" because they felt like they were a "son figure" growing up, that would be weird right?

I would avoid dad completely, since that is gendered. Having a bad parent sucks, and if you don't want to acknowledge her as a parent you shouldn't. But if you must acknowledge, I'd also avoid the genderless "parent". It feels like how people call trans women with she/her pronouns "they" as a way to deny gender. Maybe using the more formal "mother" to imply distance?

I don't talk to her much (cause she's a bigoted fascist truscum POS

Ooof. Yeah, i can see why you don't talk to her much. Still, even truscum should never be misgendered, it's a matter of principle that we should always stick to. And that absolutely includes familial terms. I can live with people using gender neutral terms like "child" for me, but it means the world to me when my mom calls me her daughter instead, and it also means the world to me that my brother, who's always been unwavering in his allyship and is just all around a cool dude, has consistently and without fail referred to me as his sister instead of just his sibling ever since i've come out (that should, btw, be the standard and it's not what makes him an ally, but always sticking up for me, correcting people who misgender me and taking a burden off my shoulder by explaining the basics of being trans to my mom after i came out do). And being called a son by my parents would sting. I would at the very least stick to using a neutral term like parent here. Calling a trans woman your dad or father is, unless she is very explicitly ok with this, universally a grave transphobic insult. That's just how it is. i'm not gonna mince words here. It's not ok unless it is her clearly expressed wish you use these terms. I get it when you say she didn't act like a mother towards you. Even if we leave out gendered roles in parenting, that requires more than being a woman who's had a part in bringing you into this world, but dang. Calling her your dad is just wrong on every level.

With that out of the way, here's some general thoughts about the subject (note that for the sake of simplicity, i'm using male gendered terms in the first paragraph that i wouldn't use for your parent, but that are fine here because my examples in that section are exclusively cis men).

I very often see people who've gone no contact with their parents use terms like Erzeuger (the term for "conceiver" in my first language, German) for their father, which is a very clear way of saying "my relationship to that asshole extends to the regrettable fact that he came inside my mom 9 months before my birth." So there's a very rough connotation of a highly strained or completely broken relationship there, one that emphasizes that person failed to provide any of all the good things our society associates with parenthood. When we reduce human interaction to matters of biology, that's never flattering. Some people have done things to their kids that make this fully understandable, i know that people who do the Erzeuger thing have very good reason to, but ... like i said, these are cis men i'm talking about. Keep in mind that for trans people who are routinely excluded on the grounds of a biologist essentialism, being reduced to our gametes is incredibly fucking hurtful, openly denies core aspects of our humanity and is a key strategy in denying trans identies. Focussing on "basic biology" is a weapon transphobes use against us. This makes talking about us in "purely biological" terms not a purely biological thing, but one that is inextricably linked to anti-trans ideology. So, that's the biological angle, and as you can see, emphasizing that aspect is at the very least problematic here.

Then there's the rainbow family angle, where people have more than one woman who's a parent to them. I see first names used a lot in such contexts, and i think using just the chosen name to refer to your parent is fine. I also see a lot of synonyms for mother, like people calling one of their mothers mom and the other mommy or nana or whatever. I also see the term co-mother being used, but that usually happens not between the co-mother and the child, but in a legal context, like "advocating for the rights of co-mothers in lesbian relationships", and the co-mother is usually not the person who gave birth to the child. But it's another term that exists. Lastly, i also know trans women whose children do just call them "my other mom", and that is a big deal to these women especially considering the fact that almost all trans moms i know have gone through incredibly messy divorces after they came out to their now ex wifes and have struggled hard to maintain a good relationship with their child(ren) in spite of the ex taking a headfirst dive into transphobia out of the sheer spite resulting from the mortal inujury to their fragile heterosexuality that resulted from their partner coming out as a woman. A lot of people can't be normal about this, can't just accept that they were simply as mistaken about a person's gender as most of us have been ourselves for large parts of our life. I don't know how the breakup between your parents played out, i want to make it clear i'm not making any assumptions here. But i've seen a lot of families break apart in awful ways after a trans person coming out and that plays into how i view these matters.

Now, every trans biography is different. That's important to stress here. I don't know you, i don't know your parents, i'm not passing any judgement beyond the basics i wrote in the first paragraph. Jut laying out every possible angle here in full to give you and the community the most complete take on the issue i can offer.

Dont deadname or dead gender her because her transition isn't why she is shitty. Its her beliefs and behaviors that suck.

Calling her "parent" is not misgendering and isn't "mom"

Man fuck truscum tho, ick 🫂

I think the best thing to ask yourself was if you felt comfortable calling her your dad before she transitioned. If you did, then honestly yeah there's probably some internalised transphobia you need to examine, but if it felt awkward before it might just be that you don't have that relationship with her, in which case using her first name is probably the best bet - I have a few mates who, consciously or unconsiously, stopped referring to their parents by their relationship.

Would you still call her your father if she wasn’t trans? Did you call her your father before she came out as trans? Do you have different standards for someone being worthy of being your mom vs your dad? These are questions you should investigate a bit. If the answer is no then you’re good. If yes then you probably have some internal transphobia and/or misogyny to work out.

Is there some reason you can't talk to her about it? Like saying, "Hey I know I haven't called you mom, here's why" or, "what should I call you?"

I'm not great at navigating difficult social situations, so maybe I'm missing something, but that seems like the best way to avoid problems.

Personally, I don't call my mom "mom" directly. She's who I refer to when I say mom to others though. I also have a step-mom and a birthmom. But that's largely because birthmom and her decided that my mom wouldn't be called "mom" to be less confusing (supposedly). So I don't think there's anything odd about not calling a parent "mom." In my case, I'm on good terms with my mom (even if she's mildly transphobic) and she's AFAB, so certainly not me being secretly transphobic.