If you're familiar with Matthews, it's probably because of the five-year, $50 million deal he signed with the Angels in 2006. Matthews got the deal on the back of a strong 2006 season with the Rangers, then promptly embarrassed the franchise by being linked to human growth hormone before the 2007 season even started. He then embarrassed himself by flopping on the field and finding his playing time shrinking and shrinking over the last three years.
Either that or you're a Yankee fan who saw him make the final out of the American League Championship Series in October. The general point is that Matthews isn't the kind of player that makes anyone forego a trip to the bathroom when he comes to the plate.
Matthews is still a strong defensive player, something that gives him some value in the expansive confines of Citi Field, and the Angels are paying all but $2 million of what's left on his deal, but it's still a head-scratcher. Angel Pagan, presumed to be the beneficiary of Beltran's absence, is a better player than Matthews and he was already here. The desire to add more depth is understandable, but you could get that for just the $2 million without surrendering a useful pitcher in return.
That said, trades for fourth or fifth outfielders aren't anything to get yourself in a rage about. Stokes wasn't going to make or break the Mets season, and one would hope that the relatively meager sum handed to Matthews won't stop the Mets from adding the pitcher they need for their rotation.