Sure, sometimes it is. When I doubt my career skills, I can (usually, eventually) recognize it as insecurity / impostor syndrome / whatever.

When I doubt my dancing skills, it's a rational evaluation.

It sounds like what you're doing is accurately assessing your lack of dancing skills, not doubting your dancing ability. A hallmark of self-doubt is the nagging unknown, a questioning, a debate that goes on in one's head as to the real truth of the matter. I'd bet such a debate doesn't exist about your dancing given the rational part you mentioned. If it's actually a correct, accepted, rational evaluation, then what you have is self-fact: I can't dance well; rather than self-doubt: I'm not sure if I can dance well. The rational evaluation part eliminates the doubt once it's accepted, as you've arrived at a rock-solid conclusion.

Yes, unless you believe that a) nobody is ever bad at anything, or b) nobody bad at something ever notices.