it just means your hair looks good today

In this situation where someone is speaking about another person's hair, "on point" means attractive or stylish. It's informal American English.

On point, means, like good point, you are making a good point, or pointing out the problem or solution exactly. So if someone makes a comment and then the other person says on point, it means like excellent point or idea. Not sure about the hair example you provided, I would need more context to explain that one.

In this context, it means - "exactly at the place where it should be " -stylish or attractive as mentioned by @Abby Teaches English

Yes, it is an American slang.