I had a close relative who tended to do this, too, OP. It certainly takes the sparkle off of things, doesn't it?
I like the PPs' idea of tallying the number of complaints, even if it is just a mental tally. I also think that the complaining would be an occasion for "So you say.'
After the first or second complaint, I would reply with "So you say" to each and every complaint. As this wore on the reply would become more like "So. You. Say." At some point mom is likely to notice your repetition and call you on it. At that point you can say, "Mom, do you realize that you have complained xx number of times since....we got here, x o'clock, whatever?"
You could then ask her if she wanted to leave, do another activity, and so forth. If she is a reasonable person at all it seems like calling this to her attention would make her want to get out of such a bad habit.