A Mary Sue isn't just a character that's based on the author -- it's one who has exaggerated traits (fancy name, unusual hair/eye color, brilliant, etc) and who becomes the focus of the story. In Star Trek fic, the example is the beautiful young ensign whom Kirk falls madly in love with, only to grieve forever when she's tragically killed. In L&C fic, I think it's more likely to be Clark's younger cousin/old friend, who solves the case and helps L&C's romance progress when they're stuck.
If, however, you want to give yourself a walk-on part, that can be perfectly fine. In Tryst, quite a few things (not everything) about Ginny were based on me and my experiences, but I trust I kept her to appropriate supporting-character status.
PJ
"You told me you weren't like other men," she said, shaking her head at him when the storm of laughter had passed.
He grinned at her - a goofy, Clark Kent kind of a grin. "I have a gift for understatement."
"You can say that again," she told him.
"I have a...."
"Oh, shut up."
--Stardust, Caroline K
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