A mug punter is someone who thinks he's good at betting on sport but is, in fact, hopeless. He can be found on betting site forums and can easily be spotted by the pathetic attention-seeking nature of his posts where he proclaims he's the greatest tipster / master of betting etc, and follow-up posts to point out the occasional winner he selects. Most of these winners are blindingly obvious and at short priced odds.
His tips usually follow a pattern of a short run of success, which promotes a feeling of invincibility, followed by the inevitable extra large bet on a loser, wiping out his entire bank. Then nothing will be heard from him for several weeks, whereupon he re-emerges having scraped together some more money for another go.