There is nothing wrong with “cool” as an informal sign of approval. I disagree with the Urban Dictionary editor who said that it’s ugly. In fact, as far as informal words go, “cool” is quite a venerable example, since it has been in use since as early as the 1960s I suspect.
The only reason I’d use “nive” — which I hadn’t heard of until now — would be if I was in the (probably very small) group of (probably teenage) speakers who use it.
And by way of an additional, slightly off-topic piece of advice: Since this group is English Language Learners, I’d recommend that you don’t rely too much on Urban Dictionary. It, and sites like it, deal with the very “bleeding edge” of English, as it morphs and changes, almost in real time. There’s nothing wrong with that, and is part of what makes English such a powerful and rich language, but it’s not the best place for most learners. As an example, take the word you ask about, “nive”. It’s not clear it’s even an English word yet and no one would fault you for treating it suspiciously and even refusing to use it.
That said, I do value Urban Dictionary because it lets me scare my kids when they find out I know slang words that they thought were only understood by them. For example, the expression on my 12 year old’s face when I told her that her eyebrows were “on fleek” was priceless! 🙂