While I agree with you on most of it, some drawing styles simply look better than others. When you look at a set like Swamps, or Corrupted Sanctuary by Disguise for example, the drawing style includes a lot of detail and is made to look realistic. They both also have a lot of texture. I'd personally choose those sets over sets which look more cartoon like, or more flat, because to me those sets look a lot better and pleasing, and very detailed. Apparently a lot of people also share my view, because both those sets are rated very high. If you look at other games now, (Including 3D ones), they are progressing to try to make them look as realistic and as detailed as possible. That's just the trend in the drawing style I guess.

I also disagree with you on the point where a badly drawn set can make as good a level as a well drawn one. The peices you have to work with have a lot of influence on your level. If the pieces are poorly drawn and don't look so good, your level will be impacted by it. An example of this is Box Sanctuary by Dax105. He used a set which got a 2.something on j2o. While he is a good levelmaker and the level he made should've been very good, the tileset he used was very poor quality, and his level got a 7 because of that. If he had chosen a better tileset, his level would probably have gotten around an 8.