That's the definition, but I wouldn't call Superman a Mary Sue; the guy's got flaws and weaknesses. A bit corny, at times too trusting, a little straight-arrow, occasionally guilty of hubris, and he's very vulnerable to kryptonite and magic.
Supes has flaws and traits that people don't like, which instantly disqualifies him from being a Mary Sue (or Gary Stu, since he's a guy).
Similarly, Katara is not a Sue either, since she has flaws and shortcomings which she has been called on about.