It's kind of cool how they kept the merch of Big Bird being a baby, or having a softer appearance, in comparison to other adult characters, like the count or Bert and Ernie!
If you really look at it though, they've kind of turned Cookie Monster into one of the more childlike characters. For example, when Sesame Street makes merchandising for Cookie Monster, Elmo, and Big Bird, they sometimes make them lighter, and have a softer, cuter appearance. This has been done before of course with other puppets in the universe, but it's usually done when turning an adult character, into a child one, or sometimes and infant.
I have a theory this change happened when Sesame Street started coming into a broader audience, and to sell merch, they had to turn certain characters more infantile to appear to the older white Grandma's so they would buy their collectables. I'm so serious. Have you seen the amount of cute baby figures white Grandmas were collecting back then? The ones you see in the thrift store is just the surface.
(Example Baby Cookie Monster)

(Old Big Bird Plush)

(Newish Big Bird Plush)

(Grandma Collectables)