Yeah, MBB has been terrible - although she's not helped by 11 and Hopper being stuck in the same holding pattern dynamic for 4 seasons now. It's amazing how little growth the characters have shown since s1 ended - it's tempting to attribute that to the Duffers having to deal with a mixed bag in terms of talent, but it's true of Hopper and Joyce, too.
The writing is just terribly lightweight, and the mammoth runtimes only magnify that.
How I see it:
S1: fantastic.
S2: a worthy sequel, that drew a little criticism for being quite so similar to S1.
S3: they try to freshen it up by going a bit lighter and goofier, with mixed success. Still broadly enjoyable.
S4: a big over-correction, where they ditch the fun and go all-in on the seriousness and making it an "event". Unfortunately, the plot is meandering and the character writing too thin to cover for that. Saved, to some extent, by some incredible high points. I've never loved Max as a character but Sadie Sink (and the excellent use of Kate Bush) saved the season.
S5: continues in the mold of season 4. Lacks the nostalgic charm of s1 and 2, the fun of s3 and the iconic moments of s4.
I rewatched seasons 1-3 so many times but I don't think I'll ever go back to s4 (at least, not in full) and certainly not this season. If they nail the finale and I do feel like a re-watch, I'll be pretending the first 7 episodes of this season don't exist.
Sidelining the Mindflayer and general Lovercraftian vibe for the super generic Vecna was also a big miss-step in terms of the horror elements.