Did Tea Party influence cause Republicans to nominate a large number of unqualified candidates? Via John Sides, Brendan Nyhan looks at the backgrounds of the current Republican candidates for the U.S. House and finds that about half of them have previously held elective office. (Political scientists usually consider previous elective office experience to be a useful proxy for candidate quality, which is otherwise very difficult to measure.) Not only is this figure similar to those of previous election years, but it's considerably higher than the figure for current Democratic candidates.
Yes, there are a bunch of inexperienced candidates with Tea Party backing out there, but as Nyhan notes, they are largely concentrated in uncompetitive districts, so they're not really hurting the Republicans this year. In more competitive races, the Tea Party has chosen to back experienced politicians.
All this suggests that the Tea Party, to the extent we can define it as a unified entity, is much more pragmatic than the media usually portray it to be. Remember that Tea Party members enthusiastically backed Scott Brown for the Massachusetts Senate seat despite his very moderate credentials. That is, he stood for basically nothing that they stood for, but they recognized the importance of depriving Democrats of their filibuster-proof majority, so they sucked it up.