According to the Pearls Before Swine strip from April 27, 2011, "It means, 'I just burned you. What are you gonna do about it?'" It's going to sweep the nation.
Rat: Dude, check it... I've invented a new expression... it's "Yo. Snap. Whassa matta hamma?'

Pastis: What the heck's that s'posed to mean?

Rat: It means, 'I just burned you. What are you going to do about it?'... I'm hoping it sweeps the nation.

Pastis: I wouldn't let it sweep my bathroom floor. Yo! Snap! Whassa matta hamma?!

Rat: It's a sad day when a nerd cartoonist gets the better of you.
by Pastisfan April 28, 2011