Let's start with this - there is no reason for pool players to dress formally. The only question is whether they should be committed to dressing presentably (at bare minimum, no shorts and a shirt with a collar). Equally clear is that the "I can't play well unless I am dressed very comfortably" argument is nonsense. Snooker players, who shoot straighter than pool players, proved long ago that near perfect play is possible even if one dresses formally.
A major producer like Matchroom will, of course, have a dress code. In regional or independent events, a dress code is at the option of the event producer, but with or without a dress code, unless the game's most recognizable players dress presentably, they give potential out-of-industry sponsors just one more reason to steer clear of pool. Mike Zuglan's Joss tour, the longest running regional tour in America, has always had a dress code, and it is strictly enforced. It certainly has improved the appearance of the Joss Tour product.
I recall discussing this topic once with a top player who was a former world 9ball champion, and the view he took was that the players will dress presentably once they get paid. Unfortunately, this is bass-ackwards thinking, for unless the players dress presentably, they will not get paid. One thing is for certain is that players who choose to present themselves in a slovenly manner have no right to complain about the lack of sponsorship in the game.
I find it comical that pro players, from time to time, suggest that they should be making the kind of money that golfers and tennis players make, but golfers and tennis players, with just a few exceptions. have carved out a very positive image for themselves by looking good and behaving well for decades. The evidence isn't very strong that, as a group, pro pool players are united in wanting to present themselves in a way that might attract the kind of sponsors that could really make a difference. Pool has failed in this respect and needs to wake up and smell the coffee.