I find it kind of funny that Fiora is meant to be the dumb one but if we actually look at the actions and statements made by the other three they come off way stupider. Meltina hid her injuries, took on high risk quests she knew she would struggle with that hit her with high penalties, and nearly ended up sold to a brothel from taking loans from loans sharks. She also neglected to do any physical therapy on the arm since the incident according to chapter 1. She also nearly got enslaved by Anastashia because she didn't listen to what she was being asked to say. Anastashia took up fortune telling and cursed a noble's son to try to make them fall in love with her when she saved them. She probably could've lived more comfortably sticking to low rank adventurer quests. Madeline poured all of her money into people scamming her without ever thinking about why things cost so much. She found out her party was scamming her and immediately barged in to confront them only to get dealt with easily after being warned that they were dangerous. She also took the bullshit Kaito said in this chapter seriously as if he wasn't just making shit up.
I guess it's a case of knowing enough to be dangerous. Fiora is dumb but the rest of them have such poor decision making skills that even if they are smarter than her they make just as stupid choices.
Has it been stated what broke in Fiora yet actually? Meltina lost her right arm and even after regeneration it had lingering issues. Anastashia had her evil spirit vessel broken. Madeline had her voice destroyed. Madeline's statement makes it seem like they were all broken in some way, but Fiora's hasn't been mentioned so far. We know she has two souls in her but that's unrelated to the hero party from what was stated.