Dangerous question.

For FEH, there is some dislike towards her because she is a contender for best unit in the game right now, and can be annoying to deal with. She also has more alts than a lot of characters that are older than her, which can bother some people.

More importantly, the form she has in the A Hero Rises event this year is Brave Edelgard, which is a unit that can be obtained through regular summoning. People want to exploit AHR to get limited units that cannot be obtained regularly, so this bothers people - especially since there is a limited version of Edelgard in Legendary Edelgard. This whole situation is reminiscent of last year's AHR, where a different, regular Edelgard form was in the running and had a huge counter-rally. So, people meme on her because she is a strong unit, very popular, and due to being very popular, potentially threatened the community's chances of getting a free rare unit, because Edelgard fans just voted for the first option on IS' (poorly organized) list of candidates.

If you haven't played 3 Houses don't even bother getting into the personality discussion, they're endless and honestly very pointless. She's a very popular character by virtue of being new and being somewhat pushed in advertising, but she's also very divisive due to her very headstrong personality and as a result, there is a large bloc of people that outright hate her. Those people get into very long arguments with the people that like her. Hard to say how much this influences the current meme trend.