What would the realistic ramifications be, if some (EU) country leader told Trump to his face that he's stupid?

Trump is talking to the Americans. He has no diplomatic skills. He would reply "so are you" and the whole scene would broadcast on Fox and watchers would cheer. It would serve no purpose.

nothing, maybe some trade shit? It would make the leader in question look worse for stooping to his level

Trump is vindictive and petty. He would take revenge somehow. Whether it is just a very STABLE GENIUS tirade on lies.social or trade sanctions or a threat to invade the country is anybody's guess. But they're all on the table and he's got the eternal memory of an elephant. That's why every leader knows they have to go in with some honey first. They pay him a compliment or bring something he wants or likes. Then they can play hardball if they have to but you gotta appeal to the frail ego first. And that's why few leadership people will call him stupid in public or even to his face. They will hide the stupid medicine in some rhetorical sugar until their country is economically and militarily independent from a Trump government.

I am no leader, praise the heavens, and I wouldn't call him stupid. He's uneducated in many areas outside real estate development. He's got the interpersonal maturity of a 4yo. But he is street savvy and media savvy. There's a reason why he is this popular with his followers.

I sorta wish zelinsky did in that rediculous display a bit ago. Lets face it, its not like trumps gonna be doing him any favors.

EU told Ukraine to go sign this deal to not upset America...

Ukraine got ambushed for a teevee show episode.

Will EU step up now or will they continue thinking US it is still US's job?

Trump would not believe it, and so nothing would be gained from it.

It entirely depends on the nation in question. Putin could do it and slap Trump and it would be ok