As I see it, you shouldn't call someone a creep merely for being socially awkward, but likewise you shouldn't excuse creepiness as social awkwardness. Doctor Nerd Love had a good write up about it.

Someone who's socially awkward is scared of accidentally crossing social boundaries and making someone uncomfortable. Someone who's creepy may or may not know where those boundaries are, but just doesn't care.

Often times when this question gets asked, the common consensus is that a creep is someone overstepping boundaries, for example not taking no for an answer. It's relieving until the term is tossed around more casually over small things.

Do you use the term sparingly, or do you use it freely about any guy that is unattractive trying to ask you out, even if he's not making you feel unsafe?