>>88700556First of all, Mania did not come from the worst arc of Venom, because the worst arc of Venom was clearly the U-Foes arc.
As for why there may appear to be an obsession with Mania, there's a few reasons. One is that I and many others do think she was memorable, that she was a really cool character with a lot of potential. Another reason is the look. Andi and Mania have this cool goth/punk look which I and others like.
The third reason is probably how Marvel has treated her. Unlike the other teen heroes (like Kamala, Sam, Miles, and Robbie) and other webslinger characters (like Silk and Spider-Gwen), Mania was basically forgotten after Agent Venom. She never got a push, never got her own mini or ongoing, and was never put on a team (though Cullen Bunn, her creator, did plan to put her in Fearless Defenders but that got cancelled before that was able to happen). Marvel basically forgot about her existence for years until Robbie Thompson brought her back in Space Knight (which also got cancelled). So basically Mania is a character that a good number of people like but has been treated pretty badly by Marvel and not given the exposure that people think she deserves. So Marvel making a new Fantastic Four with Robbie in it and putting Silk instead of Mania comes off as stinging to some, as they feel Mania would fit well on such a team and that it would have been a good way of giving her exposure and increasing her popularity. Personally my headcanon is that Andi is currently busy helping Flash track down the Venom symbiote so I'm letting the fact that Mania is not part of this new FF slide. However I do agree that Mania would fit better in a FF that has Robbie Reyes in it, because him and Mania would just go together much better in my opinion. I do hope that in the future that Robbie and Mania do meet, because I would love to see how they would interact with each other.