Onesies.
Any clothing with ducks on it.
Capri Pants.
That’s not cute on kids, either, IMHO. Frontwards and backwards are OK, in between is not.
I’ll second (or third) pigtails.
Also, freckles on kids are great. But only a small percentage of adults look good with freckles. (I say this as one of the freckled, although with age the freckles on my face have all but faded away. My arms and legs and shoulders are another story). Too bad that’s out of our hands.
Things that only look cute on lil kids?
I agree with overalls , unless you’re a farmer or engineer.
Short plaid skirts…unless you are actually in parochial school.
Shorts and knee socks…only cute on little boys at the same school where the little girls are wearing the plaid skits.
Clunking around in parent’s shoes 10 sizes too big.
Definitely - and further, EVERY kid looks cute in Osh Kosh (no brand name junkie I, but they’ve got to be Osh Kosh) overalls. It’s the first thing I buy whenever I find out a friend is having a baby.
I’m an adult who wears her hair in two braids and tends to wear overalls a lot. Then again, I work with kids. AND I work with dogs. They are more a matter of functionality than style, in my case 
SO SMEH!
In the cute kid factor - dresses that are knee-height on little girls are always cute. Not on grown women, though…
The word “lil.”
Obviously you haven’t seen how cute I am in my capri pants today, or you wouldn’t have said that! 
Trust me, it’s a hell of a lot better that I wear capri pants than shorts. Now there’s something that’s not cute!
Winnie the Pooh T-shirts and sweatshirts.
Honestly, I don’t know what the middle-aged women who wear these things are thinking.
“Look, I’m care-free and child-like! I love hunny!”
Yeah, right …