You'd have to be stupid to believe she's bisexual
Just because someone kisses someone of the same sex, it doesn't mean he or she is bi. Anyone who thinks kissing someone of the same sex automatically translates to bisexuality is either ignorant or has a secret wish to audition for the sequel to Dumb and Dumber.
Personally, I don't think Bella Thorne is bisexual at all. I think she's more likely to be fly-sexual. That's my opinion and I'm sticking to it.