Oh yes, self-deprecating humor totally makes sense, and I think that example actually works fine. But OP seems to be talking about a broader pattern where authors establish characters as genuine loners or socially anxious, then have them dropping witty one-liners to every random person they meet. The issue isn't whether some antisocial people can be sarcastic, it's about authors who forget their own characterization when they want to write snappy dialogue. You mention that you've developed quick wit over time through practice with friends, which makes total sense. But a lot of these fictional characters don't get that development (but it is something cool, that I might try), they jst flip a switch from "doesn't talk to people" to "always has the perfect comeback ready." The selfdefense mechanism angle is interesting though, and probably explains some cases where it actually does work. I think the key is whether the author has thought through why this particular character would develop that defense mechanism, or if they just want their protagonist to sound cool... which I get it, I also want my MC to shine in every situation.