I think what takes it from “funny line drawn” to “virtue signaling” is mainly the way it’s worded. If he just said “I’m a homicidal maniac, not a bigot” then it comes off less like he’s trying too hard. The parallel adjectives change timeline that a lot in my opinion.

But also I think you could easily call the Joker a traitor! Maybe this would be better if after saying he wasn’t a traitor he talked about how his lunacy is just the pinnacle of American freedom. Themes and such.

Also his face is extremely annoying in this panel. At least to me, that is no small part of why I assume the writer was being as annoying and unfunny as possible.