Truth and Honesty are better virtues to follow. If you really love someone, you will be honest with them and correct them because you want them to become better (who wouldn’t want a loved one to improve). It might be hard, it might be painful, but a harsh truth is better and more useful than a comforting lie.
“Niceness” is a way of applying truth. It shouldn’t really stand alone as that provides an opportunity to compromise truth. In light of the virtue of truth, is the question of how you act it out and apply it. Niceness can be a way of dulling the pain of some kinds of truths. Bluntness can be too harsh for people and can actually obfuscate the truth.
All things being equal, be nice. But otherwise, be truthful and honest.
Should being nice be a thing to strive for? Yes.
Is it a chief virtue? No, not even close.