It felt like they started writing her only for the benefit of Angel in the second half of season 3, once they had decided to make her his love interest. She lost all of her snark, even her relationships with other characters like Wesley, her sole focus had to be about what would make Angel happy blegh. Personally I always saw them as more family or brother/sister than anything else though. It was so ridiculous when the other characters kept pushing for Angel to express his feelings for Cordelia, and the writers seemed to forget entirely that Angel had a curse which meant finding true happiness with any woman was a really bad idea

And LOL I can't believe I actually sat through Cruel Intentions 2, what a disappointment that was!