Don't suppose you have to tell her your own experience. But I guess what's worrying her is not that she lacks snogging experience but that nobody seems to like her? Maybe she's afraid people think she's a lesbian or something? In that case, if she really is weird in some way, and you love her, you could tell her (stop dribbling! Don't paint your face green! or whatever). But if she's not weird, and it just hasn't happened yet and you're sure it will sooner or later, tell her so.