A ditzy person is someone who is silly, giddy, or scatterbrained. It is NOT a compliment, and in my opinion it is offensive. I would not call someone that, even if I knew them well, and I certainly would not want to be called ditzy.

You are silly. You are stupid. You are ditzy. You are a ditz. I suspect, as Paul comments, that's as it's most often meant affectionately, it's even more likely to be You're such a ditz. You're so ditzy.

Ditz could be offensive. Some people would surely find it offensive. Make sure you know the person well enough to call them that. Just so you know, "silly" and "stupid" are adjectives and "ditz" is a noun. "Ditzy" is the adjective form. I assume you're using slang when saying "you silly" because it should be "you're silly."

ditz is normally meant with affection...someone you know and like who does something silly...like silly while not offensive should only be used with people you know well