“Niceness” is largely performative. It’s based on words and little else. Being “nice” is based on how someone views themself.

Kindness, on the other hand, is rooted in an intrinsic belief that is shown through action. It extends beyond the individual and considers how their actions relate to society as a whole.

You can paint a layer of “nice” over an absolute garbage core personality. It doesn’t really mean anything. These days, “nice” can be used to describe a baseline level of standard civility that allows you to function in society. It says nothing about what kind of person you are.