Question

Why does ponyboy feel like an outsider

Answer 1

He feels like he doesn't belong. He likes poetry, and books and does things differently than the other greasers. He is way less rowdy than any of the over boys and he is always thinking and dreaming. Also, he is the youngest so he can't relate to everything the older boys do or experience. (like things involving relationships) 
hope this helps.

Answer 2

Because he doesn't fit - he belongs in a group called Greasers who are from broken homes, poor economic situation and little education. But he is also outsider within that group - he is younger than most of the boys, he is still in school and pretty bright.