Not in the sense of someone you can pay to get something altered. However my sister had an issue with her formal dress in 2007 and guest services when I phoned to ask for a sewing kit actually sent someone down with the kit to see if they could help. In the end they sent it to the staff tailor (who does all the costumes for the shows etc) and they tried to fix it for us but I don't think that's something that is usual.
I know when we have rented tuxes onboard (last time was 2007), we were told they have someone to alter them, etc., but I don't think you could use them for other things.
GET UP TO A $1000 SHIPBOARD CREDIT AND AN EXCLUSIVE GIFT!
If you make your Disney Cruise Line reservation with Dreams Unlimited Travel you’ll receive these incredible shipboard credits to spend on your cruise!