I honestly don't think it's a horrible shirt but there really isn't anything about it that I would think would make it fly off the shelves, and I'm right, because none of their merchandise is. Personally I would've held off on the t-shirts and stuff, first thing to do would be put them in the video game, which they've done, and see how the fans like that. Then make Nexus toys, and if the response is fair for that then t-shirts and other related memorabilia is a wise business decision.
I think they had high hopes for this not only because they're reminiscent of DX and NWO but of the entire Attitude Era and Stone Cold Steve Austin. No one's defied authority and raised hell in such a manner since Austin, until Nexus did what they did, which is why it was to be a huge thing, there's a new renegade on the block, and he has seven other renegades with him. Honestly if the storyline had been booked better and the group hadn't lost steam then yeah, they'd be making money hand over fist on the Nexus merchandise.
Changing up the look certainly couldn't hurt either, too much yellow and black. Yellow and black doesn't work like red and black and white and black did because it's too loud a color.