I know there are jokes about Czech (hodovla divek = girl's dorm, dachovy osranec = pigeon). There was a Polish satirist (Ross, Rosiewicz?) who had a song about why he couldn't possibly marrry a Czech girl because he'd die of laughter.
Oh ffs, not this one again :p
MODS!!! Thread in need of merging alert! hahahttps://polishforums.com/life/poles-find-czech-funny-23843/
Slovak is about the nearest living language to modern day Polish so they can understand that language without any trouble.
I've spoken Polish all my life and taught myself Czech, which ought to help with Slovak, but only does to some extent (I just can't get used to all that hovorit instead of mluvit stuff, haha).
Even with the help of the above, I find most Slovaks round here (there are lots) quite difficult to understand, whereas I can understand most Czechs pretty well. Even in Brno :D
My mum, on the other hand, has never attempted to learn Czech, can speak Russian as well as Polish, but understands Slovaks quite well. So how come I can't if I understand Polish at a native-like level? Following your theory, I ought to. But I don't.
Maybe the fact that even Bratislavans may have difficulty with eastern Slovak dialects may be a clue. Or the fact that the language is gradually moving away from Czech since the two countries separated. And I seriously doubt that it is becoming "more Polish".
But what would some Pole-hating trolley-pusher know anyway. :p