[Yubzzzzz]

I feel like this happened to every single character in friends. At the start, they all had fairly well-rounded characters. By the end of it - Ross besomes insufferably pretentious - Joey gets stupider and sluttier - Monica gets more compulsivly clean - Phoebe stops getting plotlines and just says bizarre things

The only character this doesn't seem to happen to is Rachel. She starts out as a naive barbie and she graduly matures into a career woman who makes her own decisions (she's also naive and barbie-ish but it no longer consumes all her lines). Reverse Flanders?

[caf]

Yes, the Friends examples are in the "Live action TV" list linked from the page: https://tvtropes.org/pmwiki/pmwiki.php/Flanderization/LiveAc...

[viraptor]

That's true, friends/Monica was the first that came to my mind. But then I started thinking about other friends-like shows and I'm surprised how much this does not affect HIMYM - there are some traits that become slightly more visible, but they're "real" and everyone seems actually aware of them. For example Joey gets stupid-slutty for cheap comedy, but Barney gets slightly more slutty and it's actually part of the story.