It's usually as simple as "hit or miss" rather than "pretty lame." Yes, the posters for Gustafsson vs Manuwa, Machida vs Mousasi and some others were roundly agreed upon as lame, but I have really liked some others that have been released in the past, and I know that everyone has their favorites.
The UFC 173 poster is very artistic and somewhat esoteric, but most of the feedback I have read has been good, which is a solid step in the right direction for graphic design in MMA. It shows us that our audience is indeed sophisticated and that we don't need to settle for keeping with the tried and true "look" of MMA: cages, explosions, etc.
Let us know what you think. We would love to hear from you on the poster and/or anything in MMA related to design.