It's just a pattern I've noticed when a character has the mc very central in their story they get disliked for it. Sure it will be couched in ad-hoc rationalizations about how the dislike is actually because of something completely justified, but the reaction can be predicted long before any of that is taken into account.
Basically people know that hating a character because their story is strongly tied to the main character isn't justifiable, but unconsciously that is the impetus for the dislike. They see a character, think all they are is some wet rag that pines over the protagonist, and that lens then colors all their perceptions of what comes after. So for certain characters people will be looking for reasons to criticize them, while for others they will be looking for reasons to dismiss them.
Another good example is Hinata, whose defining trait at the start of the show can be understandably interpreted as just being in love with Naruto. This then leads to certain flawed views about her character, they will say "Her motivations are shallow, the only reason she's trying to get stronger is because she's in love with Naruto", when in reality the reason she's in love with naruto is because he inspired her to get stronger. They will say their dislike for her is because of shallow characterization, but in reality the reason they think these characters are shallow is because they dislike her.
And yeah, there are other forces at play of course that can counter-act this, especially over time, but Tifa is exactly the same. Her characterization was never more shallow in the OG than it was in remake, but there was more room for interpretation, and because people started off not liking her, partially because Aerith exists, and because of the fact that Tifa in contrast can be easily misinterpreted to "just revolve around Cloud", people jumped at the chance to use THE WORST interpretation of her character to justify their a-priori hatred of the character.
There are exceptions of course but I am convinced this psycho analysis is essentially correct for a large part of the dislike of Tifa, if not the majority. Only reason it's changed now is because there isn't enough wriggle room anymore to justify said interpretation.