@Pebblezz I don't think Yuko fits the proper definition of a "Mary Sue" - Yuko is neither perfect, not lacking in flaws. As a matter of fact, her whole shtick is built around the "flaw" of being a lonely girl whose parents are off somewhere and she really, really wants more friends...
But I wouldn't argue about carelessness - she is incredibly dense where it comes to having sense of danger - a behavior, funnily enough, suitable of a creature that (due to her power level) has no natural predators.
And I suppose that is the point of the manga (as much as a comedy manga can have a point) - when you don't treat everyone as an enemy, you might end up having a lot fewer enemies than you otherwise might have... if you have the absolute power to support such behaviour...