Just watched the episode with Elmo filling in for Little Jerry.
I had previously thought it was odd. Little Jerry wasn't really appearing often at the time, and the group was rarely referred to by name in their old inserts. With Little Chrissy (the one of Little Chrissy and the Alphabeats) appearing in place of Chrissy or Rockin' Richard, Big Jeffy was the only regular member of the group shown, but Little Chrissy was active as leader of his own group (which sometimes included Big Jeffy), so it might have been confusing (couldn't they have just said the green Alphabeat was sick and needed a replacement?).
Recently there's been some talk over whether the names used in the "Four" song would be considered canon just because it was for an album, and recently I started wondering if Christopher Cerf and Richard Hunt should have been considered the primary performers of them, as there don't seem to be many Little Jerry and the Monotones songs to feature Jerry Nelson, Jeff Moss, Cerf, and Hunt all together as the voices (though all four of them were frequently heard in the various songs, I think Four, Sad, and maybe Body Rhythms are the only ones to include all of those performers). It does make me wonder if they actually did plan to include Chrissy, the regular member of the Monotones, and somebody mistakenly had them use Little Chrissy instead (as his name was more commonly used).
Little Jerry was the one who was sick, and they needed somebody to fill in for him. Of course the people at the dance were expecting Little Jerry and the Monotones, would they be disappointed by Little Jerry being replaced with Elmo, or do they really just care more about having a musical performance at the dance? I feel like maybe it would have been better if it was one of the Monotones who was sick (maybe say that they needed three or four members), but perhaps it's better, drama-wise, for the lead singer to be sick. Still, Little Chrissy primarily led Little Chrissy and the Alphabeats, why not just get his group to replace Little Jerry and the Monotones? Especially with Big Jeffy sometimes being in the band. And would it have been so bad if it was just Little Chrissy and Big Jeffy doing a concert as a singing duo?
Anyway, back to what I started to say before getting sidetracked with all these other thoughts I had, I thought it was odd they'd have Little Jerry be sick when I'm sure many of the kids watching might not know who Little Jerry is off-hand (I do recall seeing Big Jeffy back then, mainly from being in the background in A Muppet Family Christmas, but don't remember Little Jerry until 1994, when I was about ten). But Elmo says he's the leader of the group, and they keep talking about Little Jerry being their leader. I guess it'd play out the same if this was a one-shot group and they came up with an unseen character as the leader (after all, we don't see them perform at the dance).