Can I feel jealous of my best friend's girlfriend?
Maybe. Why are you jealous of your friend's girlfriend?
I am jealous because she is prettier than me.
It is wrong to feel jealous of your friend's girlfriend. This will damage your relationship with your friend.
That doesn't matter. I think I never liked my friend he was always a showoff and his girlfriend is cute.
Well then you have to consider your friendship. Friends will have relationships so you should be supportive of them not jealous of them having somebody that cares about him.