Is "nicer" incorrect?

I was taught a "rule" in school to avoid the word "nice" entirely. What you need to be aware of is that that "rule" is appropriate for school children who are still learning creative writing. The overuse of "nice" is not creative or imaginative which is why many teachers prohibited its use.

However, it has always been grammatically correct where any other synonym would work, and now that I am older I use it all the time when it seems like the best choice out of all the possibilities I can think of.

There are many other "rules" in English which are really style guides that have been misunderstood and applied too generally, for example never using a preposition at the end of a sentence. So, avoiding "nicer" is only a style guide, not a rule.

Feel free to start using "nicer" as appropriate!