As someone noted earlier in the thread, teenage girls are notorious for trying to mask their true intentions in order to be socially accepted with their peers; I'll add that autism, in women, often goes undiagnosed because autistic women ramp up the masking even harder than normie women. In her chats with Bella, Fiona tries to be all "maybe I can get him to stop fucking his mom at least", but would 'ironically' cosplay as Chris on Reddit and dance to Goodbye Horses on TikTok to make fun of Chris, but would then write flowery, gushy, romantic tripe about Chris elsewhere. Does she think she can help him? Is she just making fun of him? Does she really, truly have romantic feelings fo him? Was she just wanting to fuck him for clout? Her motivation for engaging with Chris changes depending on the audience.
The only thing I genuinely believe at this point is that Fiona has a fetish for gross, disgusting men (she admits this much) and that fucking Chris was a way for her to engage in that fetish the same way a foot fetishist does by sniffing feet. Anything beyond that, she's gonna have to clarify for herself.