Answer:
It's likely that Ponyboy is surprised because in their community, the distinction between greasers and socs (the wealthier, more privileged group) is very clear and strong. Ponyboy and his friends, Johnny and Dally, all belong to the greaser subculture that emphasizes a sense of identity and belonging through dress, hairstyle, music preferences, and social norms. For them, being a greaser is an integral part of their self-identity.
Therefore, Ponyboy assumes that the man in the ambulance would be able to identify them as greasers simply based on their appearance and attire. However, the man's lack of recognition shows that their world is not as divided as Ponyboy had believed and that socioeconomic status or subcultural affiliations are not always immediately noticeable or relevant.