If you had the scooter covered, how did the dirt get on it ?
Perhaps the dirt is really mildew.... from the moisture trapped under the cover.
If it is mildew, you need a cover that "breathes" and lets the moisture out.
Anyway, do NOT use Armor-All or other such silicone polish because it makes the seat very slick.
Get a soft brush and a quart of water with a spoonful of liquid laundry soap. Try softly scrubbing the seatcover.
If a stain persists consider that it may be mildew.
Scoot safely,