It [the Czech language] doesn't sound "silly and amusing" at all. Typical racist stereotype propogated by backwater peasants.
It doesn't sound "silly", but it does sound amusing to most of us. There is nothing of a stereotype in it (the word 'rasict' being completely inappropriate here), but it is the matter of a specific language similarity which make Polish people perceive the Czech language as amusing. You didn't follow the recent discussion on PF on the subject in which I quoted a once extremely popular song by Andrzej Rosiewicz with very good-natured lyrics, "Najwięcej witaminy mają polskie dziewczyny", which said:
"Czeszka tylko mnie rozśmiesza, chociaż lubię Czechów
Gdybym wydał się za Czeszkę, umarłbym ze śmiechu"
Apart from that, a once widely known TV performance engaging the top Polish singer Maryla Rodowicz and the top Czech singer Helena Vondrackova made fun of the word "laska" in a popular Czech song "To je laska nebeska" (sung by Vondrackova) which word in Czech means 'love', but in Polish means "walking stick". The performance had nothing of a "typical racist stereotype propagated by backwater peasants". No one had ever thought of it in this way.