jwojcie, if you had been travelling to the Czech Republic often and used or at least listened to Czech, you'd discover the language is not funny at all. It is just another language. No-one can decree the kind of sense of humour, yet I could describe yours as quite childish.
So, we have an understanding now! :-). That was the whole point, if I may cite myself ;) :
exposure to Czech is beyond the "comedy threshold" and you started to loosing natural Polish reaction to it ;) It is not surprisng, I know a Polish girl who learned Czech and she said she lost it to, but she said it was worth it anyway, because it is great language and culture - and I concure with that!
I mean Czechs added arbitrary all those slavic words (supposedly many Russian and Polish), and unprepared Polish ear instantly gets that something is mixed up. Well, that is supposedly the whole point of brain reaction to the joke - when things are mixed up - human either laughs or get scared... That is why learning Czechs removes all the effect - things are no longer mixed up.
So it seems we estabilished: "Czechs is funny for many Poles who don't really don't know it and even didn't heard much of it"
Now, I understand, you would alter that statement to:
"Czechs is funny for many Poles among those who have childish sense of humour who don't really know it and even didn't heard much of it."
Well, if I can take myself as an example I would say those two are the same, because childish reaction is in the essence natural reaction without knowledge on the subject. Oh boy, If I only could remove sometimes all my knowledge in areas I know deeply to be able to cheer about them as I once did... Considering the fact that in life there is less and less such things I think I will preserve my precious Czech ignorance in order to have my internal playground ;)
As for your analogy to Sweden vs Norway... You are getting it wrong. If Czechs would live on Mars and use the same language it would still be funny to childish Polish ear. It has nothing to do with neighbours antagonism.
I have no idea why do you think it is offensive toward anybody, that series of sounds makes some people laugh...