The specific critiques I saw when just skimming people talking about it don't even seem that unbelievable.
Like they said something like the game is too obnoxiously handholdy with tutorial dialogue and also they thought Luigi was basically an annoying obstacle to manage rather than a partner.
Now I haven't seen the game yet so I can't tell if that's true but that just seems like plausible things that a Mario and Luigi game might fuck up.