It's not a matter of being bland or not, it's that there was no real thought behind Mark B.'s character or the purpose he would serve. He was just a paint-by-numbers straight-white-male love interest, with a few random personality traits (he's a nice normal guy like Jim Halpert ... who occasionally exhibits sexual compulsiveness, whaaaa?!). There was no real reason behind that character, it's just that the showrunners decided they needed "someone like that" on the show because every other show has one. For heaven's sake, he technically didn't even work in the same workplace as the other characters in a workplace comedy!
Ben was initially bland too, but his character had a purpose: he was initially the show's new antagonist, as kind of an officious guy who was slashing and burning the city budget and threatening Leslie's big project. That at least cemented him into the show's dynamic, and allowed the audience to invest in him so when he did develop more personality traits, he would become more popular.