If I am recalling things correctly, someone suggested putting the Barbie movie poster as the community banner as a joke. The community liked it, and it was never revisited again.
I suppose like one other commenter here wondered: “Is it because reddit, like the poster, is outdated?”
EDIT:
I tried looking for the post where I thought it happened, found a (now dead) link to what I think is the post, but also this screencap:
Oh, ofc, this post from last year with the same question (where I got the above goodies):
https://lemmy.world/post/1252169