Is remastered mainly just less jitter?

Jitter is not only an issue of the playback but recording (digitizing) as well.  The difference is that playback jitter can be suppressed while recording jitter cannot be removed and the only way to improve it is to digitize it again from analog tapes (if they still exist) with better more stable A/D converters (clocks). Some early master tapes were digitized poorly, but there are many other reasons for remastering.  DSD, SACD, can only be mastered after conversion to 24/192, AFAIK.

"...the really early masters for CD are poor.."

Unless I’m not reading the numbers right, but looking at the DR values this theory doesn’t seem to hold up. There are lots of early 80’s CDs that are much better than then their later counterparts. I expected the opposite. Interestingly enough, many of the "popular" music like Beyonce, etc., have real low DR values. This supports the theory around the quality of (compressed) sound geared towards the ear-bud/mobile user community.

I have heard a number of remasters that are definitely not improvements over previous masters. So frequently the term remaster simply means crap shoot.
On the other hand remasters can improve things immensely, the Beatles and Springsteen catalogs are good examples.
I have no way of knowing if jitter plays a role in either scenario.

also the 2002 or so re-masters of the Stones 1980s releases are supposed to be MUCH better

Most of the re-masters I’ve heard are just louder and more compressed.

Typical example was the Moody Blues On the Threshold of a Dream, the original Decca release is so much better than the MFSL re-master.
Sure the Decca was a "little" raw, but the MFSL re-master was blunted and fat, no excitement in comparison.

Cheers George