Commercial nafness

There is a reason why I refer to Mixed Up as Messed Up.
I (personally) find there's little enjoyment in hearing random people messing about with perfectly good tunes.
My own view is that around this point in The Cure's timeline they had become suddenly fairly huge & this was something that was the product of that success.
Robert always said around the time that he never wanted to be a huge star. But the momentum of Disintegration was something he couldn't really stop.
It might be naff to some, but every step of The Cure's journey allowed new fans access to something they liked without fear of "gatekeepers" poo-pooing them. Something that often happens with bands.
MU is probably one of those moments. I wouldn't describe it a naff. More experimenting to see what the future could possibly bring. But, yes. Jumping on a train could be implied.
They also had to keep themselves on the radar fully in the knowledge that Disintegration was being lauded as some sort of Opus Dei & they had to follow that up with a new album that was at least as good, all the time keeping a growing legion of fans & music journos off their backs.
I think MU was more of a shield than anything else.

I think it might have been also their way to do something playful after having done something serious and gloomy, in a similar way to how they followed up Pornography with Japanese Whispers.

In both cases, there might have been the self-dissacrating need to tune down the aura of seriousness building up around them.

I remember reading that with Let's Go To Bed, especially, Robert was "trying to kill" The Cure, after the tensions of the Pornography tour etc.

With Mixed Up it probably wasn't that extreme, but I feel it was their way to say "don't take us too seriously".

Again, similarly, after Wish (and Friday Im in Love) made them even more of a stadium filling band, and more radio friendly (FIIL was they best ranked single in the charts if I remember correctly?), after some hiatus they came back with a "self-success-sabotaging" album (WMS) that said "who cares" in capital letters.