I think we can all agree that statements like "You are so much better than those other Xs" are always going to be problematic, however you fill in the X. ( Perhaps an exception can be made for something like prison populations - "Geez Bob, you are so nice for an arsonist!" - but let's exclude those. )
My feelings about someone complementing my parenting is to wonder if they've really gotten an idea of it. If I'm out and my children are being charming and well-behaved generally acting like little rays of sunshine and someone says "You are doing a great job parenting!" - well, I'll accept it graciously because that's what I believe is appropriate, but really they haven't seen me parenting. Hanging out with my kids when they are _good_ isn't parenting, it's a treat!
Now if DS is having a meltdown and DD is crying because he's scaring her and someone's diaper is leaking and I'm desperately trying to keep it all together - if _then_ someone came up and said "Hey, you're doing a great job" - I'd probably cry tears of real gratitude. This is when I'm _parenting_, and this is when I am doing it as best as I can. Of course, the reality is that this is when people are most likely to glare at me sideways while muttering to their companion "Geez, she needs to keep those kids under control."
So you can complement me all you want, but really if you've decided that you can tell that I'm a good parent on Monday because my kids are behaving well, then I suspect you'll decide that you can tell that I'm a bad parent on Tuesday when they are just the opposite.