Is it okay to have fun with the opponent when secured the win

example promoting all your pawns, giving up a piece, doing useless checks...

Bad form, usually. Beware you don't deliver a stalemate with excess queens. Aim to win in the most rapid, elegant, risk-free manner. I do think I got 2 promoted queens once just because the other player would not resign even when it was totally hopeless. IIRC, it was either the quickest way to get a checkmate or maybe I was making a little point. Nobody is perfect.

I mean, yeah careful of stalemate, but it is kinda disrespectful..

Some people deserve it but the vast majority doesn't and it is just disrespectful in that case.

Not sure there's any rule against it but wouldn't recommend doing it.

If your opponent was a cute animal: Would you torture it? If no, then why torture a human being?

The opponent can just resign if they don't want to continue playing so there's not really a reason to disallow it. And ofc you might mess up and still lose.

Still usually bad manners though, as others have noted.

Win and move on -- though every now and then you can cut loose

There is no guarantee of a win if "fun" moves are played, because the opponent may still have resources to exploit any weakness you have created by not taking winning method seriously.
I recall a game where my more tactically aware adversary had got my king in trouble. Despite a quick end in sight he seemed to want to finish off by clearing the way for a promotion. Given this unexpected chance, I set my few surviving resources towards his king position and was delighted to find him play a swift, aggressive, careless move that allowed me an immmediate mate.

Very rude, don't do it unless you're playing a computer or someone you personally know who thinks it's funny. I block anyone who does that to me, there's no reason to ever play them again.

I like to look for the best moves in a chess game. I don't have fun with the opponente when I reach a winning position.