I suppose I'll throw Mona a bone here, even though I actually really like Yukari:
Yukari is seen as a plain girl with limit versatility. Even her SLinks don't inspire much for creativity, and mostly ends up with her pissed off at you, or someone else. This generally makes people think she's a bitch of some kind.
From there, her entire character are in FES deconstructions her buildup from the original game, causing her to be overly pissed off all the time, and extremely jealous over the fact she didn't inherit the Wild Card, as she believed she was the closest to Door-Kun, and when Aegis got the Wild Card, her thoughts collapsed on themselves, causing her to act out with tantrum levels of anger, constantly causing people to be turned off from her in a whole.
I personally do not consider FES canon, as it achieves literally nothing outside of "Door-Kun is alive, but trapped as a door to keep the world safe, so lets not save him".
So, yes, I get *why* people think she's the worst. I personally disagree and think the above reasons are more endearing when you see everything in context, but that's just me.
