I think it's very rude to dismiss or diminish a compliment. My OH does it all the time, and has many techniques.
Contradiction
"Oh, you look great in that!"
"God, I don't. I look like a hobo."
Invalidation
"Terrific tacos! Absolutely delicious!"
"You wouldn't know good Mexican if you woke up next to it."
Unnecessary comparison
"Well done! You did a great job!"
"Nah - did you see Julie? And Angela? Man, Angela nailed it."
Unfulfilled self-expectation
"That was awesome."
"It was shit. I can do better."
....I could go on. I have identified the best part of a dozen.
Why is it bad manners?
Because it's essentially saying to the compliment-giver, 'you are not to be taken seriously, because you don't know enough for your opinion to have any value'.
The upshot of which is that, rather than have one's compliments denigrated, one simply stops offering them.