Bichão is a colloquial and somewhat informal term in Portuguese, often used to describe someone who thinks they are a "big shot" or a "tough guy." It carries a connotation of arrogance or overconfidence.
In the context of Marília Mendonça's song, the line "'cê não é o bichão" (you're not the big shot) is a defiant statement. The singer is telling her ex that despite his attempts to move on and act unaffected, he isn't as tough or as over their relationship as he pretends to be, implying he won't be able to forget her.
Marília Mendonça teams up with Maiara & Maraisa for a breakup anthem aimed at an ex who has clearly moved on fast: the clothes are returned, the photos are archived, and there's obviously someone new.
Instead of asking for them back, the singer calls it out directly. New friends, new places, kissing other people right after a breakup, none of it fools her. She's not bitter about it either. She just dares her ex to actually forget her.
The chorus, "Esqueça-me se for capaz" ("Forget me if you can"), is the whole point of the song. Changing your phone number or moving to a new city might look like moving on, but it won't erase what was real. It's a confident, slightly defiant way of getting over someone: you can pretend all you want, but you won't really forget.