A "mullet over" is the use of a mullet
wig by someone who has had their head shaved and later regretted it.
This particular hairstyle is favoured because a "mullet" haircut resembles a kind of insane wig in any case. Thus the wig is assumed by the casual observer to be an actual mullet.
If a mullet wig is locally unavailable, a deceased animal such as a gopher or beaver may be draped over the shaven cranium as a substitute.
Josh: Wow, you look crazy!
Ger: Yeah, I passed out at a party last night and my friends shaved my head.
Josh: Ha! So what are you gonna do?
Ger: Well, I reckon I'll just - mullet over.
(Ger smiles at the camera and produces a "Fruit of the Loomis"-brand mullet wig.)