These characters were not widely used but they were such a treat when they appeared. I think they were aimed at education of very young audiences since the words they used were very simple and repetitive and i think that is part of the reason why I found them so amusing then, and still do to this day.
They first appeared in a skit in 1972 where they simply appear out of nowhere to the sounds of some space age sounding stuff (well, i suppose it was in 1972 - I wasn't alive yet.) They are a very inquisitive duo and the first thing they say when they look around is "not mars... nope nope nope nope nope nope" and then they see the telephone and they don't know what it is.
The dialogue is so simplistic and their funny way of talking about how they are so easily startled is part of what made me roar with laughter as a kid. I don't know exactly when I first saw them but it was almost certainly around the early 80's.
They had a 2nd appearance in 1972 where they come face-to-face with another unknown object, this time and a grandfather clock. Of course the process repeats and they have no idea what it is and have to figure it out by talking to the clock and looking at their "book book book book book book book ."
Then, for reasons that must have disappointed a bunch of little kids, the "yip yips" disappeared for 6 years before another couple of skits were made. One of them involved interacting with Bert and Ernie, which is the first time the Yip Yips ever interacted with another member of the sesame street team. This was likely difficult to pull off because Jim Henson and Frank Oz were involved in the control and puppeteer work of both sets of characters. In the same year my favorite Yip Yip skit of all time was made.
They wouldn't make another Yip Yip skit for another 7 years after that and I really don't understand why since basically everyone loves them. I guess it was because of the limited amount of scenarios that they could get involved in because of their limited vocabulary and how they couldn't be terribly educational, which was the entire point of Sesame Street after all.
I hadn't actually even seen this last one until i went to look for it right now. I don't think it is as entertaining as the ones where they were on their own simply discovering new things and I think that the creators realized this because they would later return to their roots by discovering "books" and "wind."
The last time they were used on Sesame Street was in 2012, where Elmo delivers a pizza to space, where Elmo is clearly the star of what is going on, but Elmo kind of took over the popularity at Sesame Street during those times and probably remains in that spot.
The Yip Yips are something I will mimic in conversations every now and then in public and I am a little let down when younger people don't know what I am referring to. Oh well, I guess that is part of getting older!