Any character can be a Sue. At this point, the internet has given the term 'Sue' have such a ridiculously broad definition that almost every character falls under the category.
Having power or being lucky does not make someone a Sue. It makes them a main character. Can the Inquisitor be a 'true' Sue? Sure. But they don't have to be. A true 'Sue' has nothing to do with unique abilities or traits, but how the story bends over backwards to glorify them. Usually at the expense of other characters and plotlines. The Inquisitor does not do this by default.
I agree! It's really hard to apply the Mary Sue designation to player characters in games like Dragon Age anyways. When I think of a Mary Sue, I think of a flawless (or just has quirky flaws that are cute) character that is equal to/better than the main characters in terms of skills and likability and is implemented swiftly and easily and everyone loves them despite them not really fitting. Since we ARE the main character in Dragon Age, none of these things really apply. The story revolves around us because it has to and we have the special powers because we have to.
Now PCs can have "Sue-ish" traits. If a character is the prettiest, smartest, funniest, quirkiest, strongest, most impressive and everybody loves them, those are Sue-ish traits. And there's nothing wrong with them in themselves, but it borders just close enough to perfect to be reminiscent of what people dislike about Mary Sues. That doesn't make them the traditional definition of a Mary Sue, though, which I agree is super broad!
I never got why people called characters like Fiona Mary Sues anyways, considering she has made enormous mistakes.