I think it is safe to assume that Penelope's original interest in Junior might stem from curiosity, but I do not think it is safe to argue that her motivation for developing a relationship with him has anything to do with his culture. Instead, I would argue that Penelope develops a relationship with Junior because he does not have any preconceived expectations of her, and she feels like she can be herself with him. She trusts him.
When she first meets him in homeroom on the day he enters Reardan High, she makes fun of him.
She laughed and told her girlfriend at the next desk that my name was Junior. They both laughed. Word spread around and pretty soon everybody was laughing. (59-60)
But then, on Halloween, when she and Junior both come to school dressed as homeless people, she confides in him that she's raising money and awareness for homelessness. He asks if he can help her and she reluctantly agrees:
Penelope stared at me. She studied me. I think she was trying to figure out if I was serious. (78)
Their friendship and relationship is officially sealed, however, when Junior waits outside the bathroom door while Penelope is throwing up inside. It is clear that this is a secret she is hiding from everyone. It is also clear that because Junior treats her differently than others, she trusts him.
But Penelope starts crying, talking about how lonely she is, and how everybody thinks her life is perfect because she's pretty and smart and popular, but that she's scared all the time, but nobody will let her be scared because she's pretty and smart and popular. (108)
Penelope's interest and motivation to befriend Junior comes from something much deeper than curiosity about Junior's culture. She is motivated by loneliness and a desire for a real friendship. Because of her popularity, she is under intense social pressure to continue to be pretty and smart and popular. As a result, she is lonely and insecure. Junior doesn't treat her like the kids who have grown up with her though. He hasn't known her as long. Therefore, he is willing to accept her for who she is without expecting anything more. And, it can be inferred that because he is so much different than everyone else, and always has been, he isn't just using Penelope to feel popular himself. Penelope trusts Junior and feels comfortable with him.