I find the opposite to be strange. At least here it can be explained. Title becomes more popular = earns more money. Earns more money = can afford better illustrator. But when LN has a kickass art, and the manga adaptation is at the level of mass-produced chinese webtoons, that's when questions start to appear.
It was never good in my opinion, but it was enjoyable, but the further I went the less enjoyable it got until it just became a chore to read. Personally I have fault with illustrations because jp novels are always so light on prose that the illustrations are some of the only time you can get a detailed description of characters or scenes