I don't think the issues with the GamePad's usefulness turns people off too much. I agree that most games just use it as a mirror or inventory quick reference, but I think the GamePad is an overall minor part of a great system. Part of the marketing failure is making everyone think the GamePad is fundamental to gameplay instead of complementary and that the Wii U is some weird hybrid tablet-console thing. They've even failed to communicate this to some game developers who apparently feel like they have to make some contrived GamePad level.
I agree it was a somewhat slow start out of the gate with games, but at this point it should be selling like hot cakes. We have a really big holiday season coming up and a few of those games have already released (Hyrule Warriors, Bayonetta 2).
Don't get me wrong, I think the Wii U is a console worth owning, and right now, it probably has the best games library of all current-gen TV consoles. I also think that poor marketing and misguiding naming play a role in its poor performance — why not call it the Wii 2, to clearly show that it is different and new?
But I don't think that its failure can be attributed solely to marketing.