"And, and, after they wrecked my wagon, they made fun of me! I lost everything and they, they acted like I was the bad guy! Waah!" "There, there. Let it all out, dear." "I just wanted to entertain them..." "I know, Trixie. We'll think of something. I'm just glad you're okay."
Just because you don't like a character doesn't mean they're a Mary Sue. And just because a character has very, very strong abilities (that she earned with years of study and being predestined to be the Element of Magic; her special talent), doesn't mean she's a Mary Sue. She definitely has faults, and just because those faults are quirky and lovable doesn't make them non-faults.
Beautiful piece - I'm totally in love with your linework in particular.
I'm not sure I really see this as in-character for Trixie (although it could eventually happen), but it's VERY much in-character for Celestia; she's as magnanimous as she is powerful, as kind as she is wise, and she'll always be there for anypony who needs her support, even if they've had a checkered past. And let's face it, despite everything, Trixie isn't a bad pony, and a bit of support at the right time may be just what she needs.