I'd say it was in Menage a Troi
. The viewers never took them seriously before that and for good reasons. However, it was in this episode where the writers finally began to use them as simply comic relief.
All of their appearances up to then were The Last Outpost
, The Battle
, Peak Performance
(in all three the writers clearly meant for them to be taken as serious villains), The Price
and Captain's Holiday
(in both the "comedy" is easily seen, but they're still supposed to be the villains).