That's a tough criteria specified by the OP, because even when Star Trek is trying to do comedy, it's often trying to do something else as well. Probably the best known comedy Trek has ever done is The Voyage Home, yet it was clearly also trying to make social commentary and put forth a message.
Also, did TNG really have straight comedy episodes? TOS obviously did, with entries like "The Trouble with Tribbles" and "A Piece of the Action." DS9 had tons with episodes like "The Magnificient Ferengi," "Who Mourns for Morn," "Badda-Bing, Badda-Bang," etc. And Voyager had ones like "Bride of Chaotica." But right off the top of my head, I'm having a hard time think of TNG episodes that were just comedies.