Hi Kammy! Going on from what JJ said, a Mary-Sue can a little too perfect because it's a character based on the writing themselves or more likely, who the writer would like to be so the fanfic becomes an expression of wish-fulfillment. Personally I see nothing wrong with this as it can provide a charming little window into the writer's personality but no-one could be called unjust for finding it a little distasteful.
Really just write what you enjoy writing. I've never read a fanfic that didn't have something in it that was cool. Well...maybe 1 or 2 but I'm sure you wouldn't write that sort of thing.
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