Just a theory that her cutie-mark shows that she is able to hide her magic from such magic-stealers like Tirek... And so... When everypony will be drained out of their magic, Trixie will be the second most powerful creature in Equestria after Tirek! Brilliant evil plan!
May I point out that Trixie still has her cutie mark, thus proving that her power is indeed great. Tirek, unknowingly, had not the strength to take it from her! The Great and Powerful TRRRRRRIXIE can even resist what Twilight, in possession of all alicorn magic, could not!