Yes, I have seen some MLS trading cards. They used to have some on www.mlsgear.com but I just looked and they are not there anymore but maybe you could find some on other trading card websites.
Try the Memorabilia and Collecting forum here: https://www.bigsoccer.com/forum/forumdisplay.php?s=&forumid=273 There's some info on cards and I'm sure folks who can answer any questions you might have.