is it normal that my bf left me for a girl who is not better than me ?

Judging by the way you type, and how jealous you're coming off, he probably just got sick of of your shit and wanted to go for somebody else.

Anyway, he's the only person who can determine if she's better for him or not, that's HIS opinion.
Perhaps she's more intelligent than you? Maybe he thinks she's funnier? Maybe he likes her appearance more than you?

All of those are possible, and more. Anyway, it's completely normal.