More productive than both Lampard and Medina. Not even close
I think it's actually pretty damn close and I can see it argued any which way.
Lampard, 15 goals and 3 assists
Mitrita, 16 goals and 4 assists
Medina, 23 goals and 9 assists (add the intangible defense and MLS Cup championship)
Both Lampard and Mitrita had their own issues that led to lack of playing time (both injury prone, Lampard's late arrival and Mitrita's early exit).
Lampard played on the worst team in club history that only had Villa and the production there was down (reasonably). Mitrita played on one of the best teams in club history and his production there was way up. I could see this point being argued either for or against either player.
Mitrita didn't play defense at all. Lampard did. Medina did better than either of them at it.
Mitrita didn't combine well in the offense in the build up. Lampard did. Medina struggled but I would say was better than Mitrita at it.
Mitrita could take guys on 1v1. Medina and Lampard couldn't.
Medina and Lampard both appeared to be exemplary teammates. Mitrita hasn't been specifically called out as the issue, but he was one of the players that left and during the offseason, many statements were made by players and coaches that basically said it was good a player or two were gone (I could easily see Mitrita being part of that).
I get it if you think Mitrita was a more productive DP than Medina or Lampard. But let's not act like it's not even debatable.