It's a harmless stereotype about mexicans liking tacos, at the worst, with a Juan pun.
Love the South Park reference.
However if I was to dissect the joke (which always removes any humor that might have been present to begin with), I would say: First of all, everyone loves Tacos. I mean what is not to like? The joke is a play on words; "said no one ever" sounds akin to "said no
Juan ever" and since the name "Juan" is a spanish name and Tacos are a traditional delicacy introduced to North America from its' neighbor in Mexico....the pun doubles. Not sure how this is demeaning, even if for some reason a person doesn't like tacos (soft or hard tacos, doesn't matter, both delicious, especially with sour cream and/or guacomole), they shouldn't be insulted most people do or that people from Mexico also enjoy tacos.
Probably a more appropo South Park youtube link would have been Cartman doing J'Lo singing "Taco Flavored Kisses". I will refrain from posting that youtube link so as not to offend the fainthearted.