Funny, I was just reading about how non-German residents of Germany use "Kartoffel" (potato) as a perjorative for Germans in Germany. I didn't realize "potato" was such an effective insult it would exist in another country too!
Ostensibly the term is meant to imply that the players there have about as much capability of thought as a potato.
Wat een aardappel! "What a dummy!"
(... which nobody gets.)
Same in France with "patate". Funny to notice that the more common French expression for "potato" is "pomme de terre" ("apple of earth"), just like in Dutch!
Though in the south (both south NL and Flemish Belgium) they're called "friet", as in "gefrituurd" ("fries" as in "they're fried"). Still, patat is officially Dutch and according to some website 95% of the Flemish people and 100% of the Dutch people know the word.