I feel like it’s a play on an old phrase. I’ve heard toupees being described as looking like a dead rat.

It’s a rats nest.

I guess he still has hair, it just looks awful.

Because people are self conscious about going bald.

Because back in the day, minoxodile was said to contain rat urine, and its main selling point was that it regrows hair.

I think it’s just a joke on how far he’s fallen.

There also may be some other shit that happened after the hair ascention and the last panel.