Actually, even adults can be cute. Originally that's the lovely thing about yukkuri. When the yukkuri are getting joyed for the simple things they have. They get depressed, but stupidly cheer up again soon after. Yes, that innocently happy side.
Where the line is drawn is usually when the yukkuri starts to demand more than what are given, which more often happened when they have human around.
That's why I prefer yukkuri owner who are just observing them and keep contact to minimum.