Let's apply what JC is saying to painting, perhaps this will make it easier for you.
If I hand painted a picture perfect copy of Da Vinci's 'The Last Supper' that was ten times the size of the original, you couldn't possibly argue that I wasn't a skilled painter without looking extremely ignorant. Similarly, if I couldn't paint at all, but I drew a really scratchy drawing of a somebody using crayon, you could still argue that I was a good artist. Even if I don't have the technical skill required to draw a supersized copy of 'The Last Supper'.
Technical skill and artistic vision are not the same thing, as technical skill isn't something that is subjective.
This applies to Sufjan Stevens in that technically, his records are meticulously arranged and massively intricate. In this sense, he is undeniably a good musician. His records ARE catchy, in that they are immediately accessible to the casual listener. That's what 'catchy' is.