Yes, it is impolite and brutish to dismiss fine art from Van Gogh, Salvador Dali, and other esteemed historical artists, because they have made their mark historically. Even if you want to dismiss their art, you have to recognize the cultural impact that they have made on criticism of art and the art industry.
Art is one of the most subjective things on Earth. Some people may like something while other do not like the same art. I think that is something that will always be a part of humanity. Just because a lot of people think artists like Van Gogh, Salvador Dali, etc were good doesn't mean it's a fact.
It might be impolite by the way people dismiss the art. If they start cussing up a storm and throwing things at the pictures, then that is not only impolite but rude. If they calmly say, "I think other artists are better than Van Gogh, and Salvador Dali." Then that is not is impolite, but their personal preference.
no, it is all in what you like and is all in what your tastes are. A lot of people would not want to see these peoples art, and would think that it is not all that good, but some would pay millions just to have a work from these.
Van Gogh and Salvador Dali specifically get criticized as abstract artists. The fact is, that these artists just saw the world a little differently, and had the ability to illustrate it. If a viewer of the art wishes to dismiss it, ,that is their choice. There is nothing impolite about dismissing them, if done politely.