I totally agree, at the time I definitely felt like it was season 3 that started mishandling Cordelia's character and trying too hard to push her as the wise Saint (because really where do you take a character from there?). Some fans got peeved at Skip later laughing at Cordelia being chosen because 'she's such a pure and radiant Saint, please ', but that's a retcon that actually totally works for me. It is very Cordelia to fall victim to her own pride, she was always very into this idea of her and Angel being special Champions set apart from other people, there are so many examples of her giving speeches to that affect, so the powers came up with the perfect plan to appeal to her pride and make it the tragic cause of her downfall. It's actually way more consistent with Cordelia's character than some of the stuff they were trying to pull in late season 3, the retcon makes perfect sense of the show trying to play it straight that Cordelia is too good for this world and ready to ascend to the heavens

It seems like the sex with Connor is the big black mark against Cordelia's arc in season 4 for many viewers, but that's ignoring the fact that it was never Cordelia in control at that time