Yes, I was not that satisfied with it either, BUT I have no clue about Mortarion, to be honest. I have not really read anything with him so far, he only made side appearances and when he did, he was salty as fuck and generally just annoyed by everyone. Even during non-Horus Heresy times.
It's also hard to describe them with only one or two sentences each, if it isn't Fulgrim or Alpharius or the furry fuck. Those have major character traits that make it easy. Also, I'm fairly sure that Mortarion joined Nurgle because Nurgle was the only one that ever loved him. Nurgle loves everyone.