DS9 might have been intended as a joke, but as soon as you put Klingons without ridges back into Star Trek with a Klingon that has ridges, there will be questions and they will require an answer. If they never did that episode, no one would insist on an answer, but people would still be curious if something happened to the Klingons to make they go from looked like they did in the 1960s to how they looked in the 1990s, particularly Kor, Kang, and Koloth, who we saw in both versions on the makeup. Without Trails and Tribbleations, it could be assumed that the "updated makeup only" excuse was the truth and that "Klingons always looked like that". But as soon as you show smooth headed Klingons again, you have opened that box to the question that now has to be asked, and eventually people are going to want an answer....including the writers. Therefore, ENT eventually goes the route of telling the tale of how the Klingons lost their ridges and implied they would figure out how to get them back around the time we start to see ridges again in TMP.