I respectfully disagree. If they went authentic, people would love it. Who doesn't like quality authentic Mexican food? I think most Americans and Canadians (a huge portion of
DCL guests) are more adventurous than DCL gives them credit for. At WDW, some of the more "adventurous" restaurants are the hardest reservations to get. The average city in both countries has lots of international options now, and the people I know love to try new things. I don't think the average American just eats hamburgers and Taco Bell these days. I think the idea that we have simple pallets is outdated. The first thing an American or Canadian wants to do in a foreign country is try the local cuisine.
Even if they went American style, there are plenty of options that don't involve boat loads of cheese, unless you are doing it like those 1990's style places that are falling out of style. There is an upscale Mexican place near us that does plenty of Americanized versions of dishes that aren't loaded with cheese in a style that would fit the MDR's perfectly. Really flavorful stuff, plated well, but appealing to the average person. You can't even get into the place because it's so popular, and I live in a pretty homogenized American city.
Even if they use cheese in many dishes, they could have options without it. But the majority of people can handle cheese, even if a handful need to take some a pill first. People love cheese. Done right, the American versions can be pretty good. But the quality varies a lot.
I think a Mexican restaurant is exactly what DCL needs to liven things up. It's truly one of the best cuisines in the world and is glaringly missing form the ships.