Nowadays, it's pretty standard for a desperate guy in a movie to shave his head or give himself a hairstyle that involves shaving at least part of his head—think Richie Tenenbaum in The Royal Tenenbaums. That's largely thanks to Taxi Driver.
When Travis decides to assassinate Charles Palantine, he shows up at the rally with a Mohawk (almost definitely a Mohawk he gave himself—it doesn't look like a professional job).
In Travis's mind, this new haircut probably signifies how he's been "reborn" as a cold-blooded killer. He's done with human connection and the search for love. Now, he's just an appetite for destruction. In this sense, it's kind of a parody of the way Buddhist monks shave their heads when they're entering a monastery—a way of symbolizing being reborn, since infants have hairless heads.
An actor friend of Scorsese's, Victor Magnota, actually suggested the Mohawk cut for Travis, based on the way certain soldiers in Vietnam cut their hair sometimes, when they were planning on going into crazy commando situations. This is equally appropriate since Travis is a Vietnam vet.