Went digging through my holdings and found I had a mono MMT EP (Electrola, Germany pressing) from probably the late 1970s. Checked it, and it's clear of that phasing effect, so no, it wasn't an intentional effect.
My best guess is that the mono master tape was transferred using a multi-track machine with the heads slightly out of alignment. When the tracks were reduced down to mono - voila, instant phasing that should have been caught by quality checkers doing comparison against other vintage sources. *But* that's only a guess.

Given how good the stereo version of YMSK sounds [and despite being a different mix they worked from largely the same individual tracks to create the mono and stereo mixes], I'm going out on a limb and say it wasn't intentional in the sense that The Beatles wanted it to sound like it's "under water" (which I agree it does on the mono album).
Rather, since the people who worked on this were adamant that they were only remastering the best available master tapes and not going back to the original instrument tracks to create an entirely new mix (or attempt to faithfully recreate the mixes forty years later) I'm going to guess that the best mono master they had of that track was simply not in good condition and they did the best with it that they could, and we got the result.
I can't imagine they worked on the catalog as long and as diligently as they did only to royally mess up that track. I'm guessing the original master they worked didn't survive the test of time very well.





According to Allan Rouse, EMI's supervisor for the Remasters project, the mono mixes were transferred off an analog tape machine using a dedicated mono playback head of original vintage, so your speculation regarding the phasing problems on "Your Mother Should Know" are incorrect, Dave!





