Your example can be explained by one name: Sayger.
His style was all his own, and it's all a matter of taste in that regard--not a matter of quality.
Post-Sayger, the colors were actually really good, I thought. Leigh Gallagher's art reminded me a lot of Carlos D'Anda, my favorite artist, in that the colors were so rich, and the characters went back to being based more closely to the sets. Pop Mhan was similar in that regard.
Sayger's pretty much the only one. He's not someone I would have wanted doing the entirety of BIONICLE's comics run, but he was well suited to the darker tone and return to mystery that was brought to the theme in 2006 and 2007. Any other years, and I think his art would have been out of place.
To answer your question, yes, the quality did become poorer in later years, because not so much effort was put into the line after 2005.
I don't think that was the case at all. If anything, more effort was put into BIONICLE after '05, particularly in 2006. Story serials, BIONICLE Heroes, Voya Nui Online Game, Glatorian Arena, the invention of the Inika building parts, the eventual abandonment of playsets in favor of Technic vehicles (which, in my opinion, are superior, and at the very least have the right masks on the right characters), and of course, The Legend Reborn. Sayger's art was a stylistic decision--not a lazy, money-saving one.
Edited by LewaLew, Jun 14 2013 - 01:51 PM.