In the film, Vanellope is a 'glitch', but...
... she's only a glitch because Turbo rewrote her code...
...which should have been 'fixed' on crossing the finish line.
Why is she still a glitch?
The "glitch" was actually a legitimate in-game power that Vannelope possessed. It was only because the Sugarlanders had been 'memory wiped' that they didn't remember it (and hence made fun of her for it).
Note that final arcade scene at the end of the film, we see the young racer activate Vannelope's glitch using the horn button. At the top of the screen, one pink diamond is removed to indicate that a special ability has been used. Presumably each of the other drivers also have in-game abilities.
And confirmed in Ralph Breaks the Internet
Girl: Pick Vanellope, Bonny, she's got the best superpower!
Bonny: Yeah, the glitch!