If any given act is "JUST NOT THAT GOOD," they shouldn't be scheduled for multiple appearances at an event where people pay an admission fee in order to be entertained!

Some degree of professionalism should be expected, which would include an awareness that each time a crowd gathers for your festival appearance, some of them might well be "repeat offenders" catching your act for a second (or third!) time. If you can't offer an entirely new and different set list each time around, you should at least be able to vary the order of the pieces in your limited repertoire and, between musical numbers, to avoid repeating memorized banter in a word-for-word fashion!

However, "Guest, meself," I do agree with you where you say that if...

...you find they don't stand up to a second listen and you're disappointed - it's nothing to get worked up about; just cross them off your "must-see" list and move on.