After listening to the roar of the crowd all weekend you would think he is. I say there are two ways to go with this question. Yes and no. Do the fans love to root for him, you bet they do. Is he becoming more popular because Tiger is in a "slump", yes. So is he the most popular guy out there yes, until Tiger starts playing like he was a few years ago.
Part two to this question is: No, Phil is not the most popular guy out there. If he were the most popular guy on the PGA Tour he would have much cooler sponsors. I'm sure he is paid a pretty penny by Boiling Point, Ford & Titleist, but who really wants to wear a visor with Boiling Point on it? Only a guy getting paid to wear it, that's who. I'm familiar with B.P., but I would never wear a visor with a big Boiling Point all over it. What it comes down to is that Phil is liked by the fans, but Phil doesn't sell. His fans wouldn't go buy a shirt that has a PM or a Phil on it, yes they'll play Titleist balls, but that is not because of Phil, that's because their Titleist balls. Heck, I don't think even Tiger is selling many of the Nike balls because of the ProV1's, but you see his shirts and other golf gear everywhere.
So here it is: Yes, Phil is the most popular guy on Tour, but he will make a 5 foot downhill breaking putt before he will ever sell a Phil Mickelson endorsed shirt.