Yes, there's an element of self-fulfilling prophecy about it, but there's also acceptance of the inevitable. I know a few servers who I trust would remain professional and courteous to groupon customers. Universally complain that groupon users make poor customers. (Not strictly limited to groupon. "$2 specials everywhere" bar crawls of any ilk draw similar complaints.) I mean, it's not like all the servers got together and decided to A/B test groupon customers by being meaner to them than others just for the funsies. How does a server with no priors come to treat one group differently than another?

It's less self-fulfilling than you would expect. Research shows that tip amount isn't very affected by the quality of service, but much more affected by the person doing the tipping.

If you walk down a street and nine people punch you in the gut as they pass, it's a bit rich for the tenth person to complain that you flinched as they walked past. The wait staff are simply playing the odds.