In this phrase: "Yo había perdido la pasión por temas de orden menos digno", we can use the variant "poco" for the word "menos", and like this it sounds better, I think. But the first phrase is wrong?
The sentence is grammatically correct. "Temas de orden menos digno" means "topics of a less dignified order", or more simply, "less dignified topics".
For this sentence to make sense in context, there should be a basis for the comparison that is implicit in the word menos. That is, a certain topic of thought or discussion must have been mentioned before, so that these "less dignified topics" could be said to be "topics (that are) less dignified than" that previous one.
You can use poco here as well, but then there's no comparison. "Temas de orden poco digno" just means "topics of little dignified order", or more idiomatically, "scarcely dignified topics".