I like Cordelia a lot in Season 3, but she isn't one of the most important features I think about when I think of S3 either. She's away with Groo for several weeks, the Connor/Holtz plotline becomes the dominant issue for most of the season, etc. There is of course the part about Cordelia becoming half-demon and setting into motion the events that happen in S4, and great episodes like Waiting in the Wings. I can understand how some people would prefer the funny, shallow Cordy from earlier years(I like her too), but I love mature, selfless, heroic Cordy most, and that really is Cordy from S3.
Part of what puts people off about Cordy in Angel I think is that they try and change her too much and so lose what made the character so good in the first place. As a result maybe it is hard for people to believe that it is still the same Cordy.
From what I've seen on the internet, it is Angel S4 where fans resent what is done to Cordy's character the most.
As far as the issue itself you point out, I love characters who continue to grow, evolve and change. Cordy grew and matured a lot on Buffy, but why would growth and change stop there? Growth and change
accelerates after high school, not slows down. I agree, Cordy is very fun as the shallow entitled princess, but every period has its time and ends eventually. There are many characters I really like as they are in specific periods, but I have no problem (just
in principle, if not in specific execution) with things changing suddenly anyway or gradually about their characters, because the only constant in life is change.
Fred changing into Illyria isn't a great example of what I mean (because Fred and Illyria are two different people) but I loved the idea of the evolution of that "character" from Fred into God-in-Fred's body-with-her-memories, etc. And I
loved Fred as a character; she was one of my favorites on either show. But it was a great idea (the idea of Illyria was intriguing to me) and it stirred things up and kept things from settling down into a predictable state. This is more important to me than having reliable and consistent characters (characters who are more consistent than change) so I never saw Cordy's change, development, evolution, whatever you want to call it as anything but interesting and a welcome change, because I think they did it well with regard to Cordy (well, until S4...I didn't like what they did to Cordy S4).