It was interesting how Clara went from one of my favorite companions when paired with Smith to one of my least favorite with Capaldi even though I loved Capaldi.
I do think that's indicative of the writing taking a shift for the worse.
How strange, I go completely the other way. Clara as the "Impossible Girl" was bothersome, it was another annoying mystery which didn't do much for me. And the romantic tones between her and Smith frustrated me. Clara was at her best with Capaldi in her last season, I didn't care for Capaldi's first season that much.
I haven't noticed a downward trend in the writing, but I've noticed a flattening. Early seasons of nuWho have had superlatively fantastic episodes, but also absolute bombs. The last Capaldi season, and the two seasons of Whittaker have been pretty evenly good for me. Nothing has captivated me as much as say "Blink", but nothing has frustrated me as much as "Fear Her", for example.