What does "headass" mean and why do kids say it?

I assume that "head ass" is either short for accusing someone of having their head up their own ass (figuratively, of course), or saying that they have a face/head which looks like an ass.

Believing that trendy words are "becoming more weird" is a sure sign that you're getting old.

Young people have always used slang to bond and exclude older people. Sometimes the slang sticks around and becomes part of standard English, and sometimes it fades away.