Get it? Because there are hammers involved in Marvel's big summer publishing event? Isn't that ... funny?
MC Hammer was the stage name of Stanley Kirk Burrell, and left his indelible mark on pop culture in the days when old school rap was simply just called rap. Those pants and signature shuffle were most popular with 1990's "U Can't Touch This." That only makes this reference 21 years old!
In case you are a teenager reading comic books, here is the music video the cover artist wants you to remember:
Dated, obvious, tired and cliche.