Compliments are hard to take for some, and it sounds like you find it patronizing and creepy. This is for this very reason that I was really hesitant as a woman to compliment men, I'm more or less the same generation.

One time late at night, I had no makeup on and was very frumpy, going to buy a six pack at 7eleven. There, a gorgeous gigantic drag queen told me "Giiiiiiirl, look at your faaaace, you're so gorgeous". It was so so so cool and made me feel like a queen. So from then on I thought fuck it, I don't get compliments often and when I do, it makes my day. So now I do whenever I am sincerely impressed or enjoy something.

At some campgrounds you can't not hear your neighbors. I understand there's a social expectation of pretending privacy but surely this is just a wholesome gesture?

Why?
Seriously, it seems like a genuinely nice note. No harm was done and everything that was said was validating and positive. It didn't need to be said, but that just makes it all the more special. That guy going out of his way to give that validation when it obviously wasn't necessary just shows that he isn't just being a good dad. His parenting is at a quality at that at least one other dad admires. He doesn't need to be told that, but I'm glad he was.

I suppose I don't understand your perspective. If you know you're a good dad then getting validation on top of that is just good, right? If it's obvious you don't need it and someone else is still compelled to tell you then you must be doing an even better job than you thought!

It's good you don't need validation. In fact it's a great level of confidence in your ability. So if someone validates that confidence. That's good? It's not needed, but it's still good.

Or at least that was my takeaway, I was curious about yours.

ETA: I had a thought... What would your opinion be if the feedback was negative? What if he was aggressive and mean? Not loud, or drawing attention, just a dickhead to his kids and family.
I'm not looking for any particular answer here, I'm just curious if your opinion would change at all.

the weird part to me is writing such a long note and putting it on the car.
if it was delivered in person (and then there's still a lot that could make it weird), or the note was just "hey man, couldn't help but notice you seem to be a great parent, props from a fellow camper", i'd be more comfortable with it.