TL;DR: This quote sums it up: “At his worst, Elmo encourages immature behavior rather than discouraging it.”
From the article:
At his worst, Elmo encourages immature behavior rather than discouraging it. I remember a storyline years ago that perfectly encapsulated this: Episode 3280, Season 26. The storyline begins with Gina’s boyfriend, Jesse, reading a story to Elmo and his friends. After Jesse is finished, all the kids want to take the book home. But, luckily Jesse has a copy for each of them. It was a cheap dodge to a teachable moment.
Later, Gina and Jesse plan a picnic date. Elmo wants to tag along—in fact, he assumes he’s tagging along and gears up, without asking for permission. Gina tells him that this picnic is for grown-ups. Elmo is visibly sad and downcast. He guilts the couple. So the boyfriend cracks and lets Elmo tag along. No gentle reprimand. No establishment of boundaries. Just a full-on, unreserved capitulation to Elmo’s wants and needs.
“YAAYY!!”
What message does all this send? It’s easy to imagine Gina telling Big Bird, “Sometimes, Big Bird, adults need time too. If you want to go to the picnic you should ask for permission.” Nah. Elmo’s method is better. Intrude on personal space! Whine and quiver your lip, kids! Guilt gets things done! Cuteness will get you everywhere!
Source: How Elmo Ruined Sesame Street