ID Sci-fi Short Story: The Loopy's Dance

Something just reminded me of a short story I read ca. 1968, about a sweet naif who’s driving around with a group of futuristic juvenile delinquents. One of the kids suggests they go watch the Loopy dance. They get to the club, and the Loopy (the word is formed from initials) turns out to be someone horribly mutated by radiation or somesuch sci-fi bugaboo.
I barely comprehended what was going on then, and I’d like to read it from a more educated perspective.
Does this one sound familiar to anyone?

Dance of the Dead by Richard Matheson


Short Story it’s apparently in I Am Legend, but I’m not good at getting amazon to show that.

Thank you!