I agree with you that Robin should not have been in the series, but if you're going to be that technical, then Scooter shouldn't have been on the show either, since on The Muppet Show he was just a kid when the others were all adults. Realistically, when Kermit, Fozzie, Piggy and the others were babies, then Scooter shouldn't have even been a gleam in his daddy's eye. For that matter, the whole series didn't make sense, if we're to believe that the story of The Muppet Movie was how they all met, but...
....getting back to the subject: Skeeter was just an OC who was added to the show so Piggy wouldn't be the only girl Muppet Baby. Having witnessed NBC catch flack over having only one female Smurf (Sassette and Nanny Smurf wouldn't be added 'til later), they weren't about to let the show go on the air with just six boys and only 1 girl. Why they didn't just go with Janice or Miss Mousy or some other established female Muppet I have no idea.