I'm having this issue as well, and it's incredibly annoying. I've gone through the daily mixes multiple times and I've constantly had to click "Don't play artist", on the same artist multiple times. This is not just on features, but on songs that are soley by that artist. I wish there was a way to delete a daily mix completely and allow it to repopulate based on what I'm actually listening to rather than loading up playlists with songs & artists I've no interest in, and no way to change.
Long story short the "Don't play artist/song" functionality does not work on Daily Mixes. The content is stale.
This is just speculation but could it be that the dislike function has more effect on discover weekly? It seems to me that the daily mixes are far more themed than the weekly so maybe the threshold for getting somthing out of your daily mix is harder than on your weekly?