I think the idea behind Romney choosing Ryan as his Vice President was to excite the conservative base that was not all that thrilled with Romney in general, I think it was a strategic move on Romney's part to do this but in the end his opponent just ran an overall better campaign then Romney and it was never really a close race.
Firstly, you had a flip flopping Mitt Romney who struck a lot of people as a sociopath. He was the governer of one of the most liberal states in America, and passed healthcare for all in the state. Paul Ryan, on paper, is the complete opposite and a draconian conservative who pretends that government shouldn't exist, even though he's a politician.
Paul Ryan was not a good running mate for Romney because there was nothing particularly memorable about him. He was a conservative young white male. America is at a period in which people want politicians to look like them and understand their needs. Therefore, Ryan did not add any popularity to Romney's campaign.
While Mitt Romney's choice of Sarah Palin is widely seen as a major mistake, equally mistaken would have been the choice of Paul Ryan. Both of these choices imply that Romney had to pick a running mate who would have demonstrated his conservative bona fides, but this was mistaken; he should have played to the center in order to appeal to the largest possible constituency, rather than trying to steer support from the far right.
Paul Ryan was the picture boy of a perfect Republican candidate from twenty years ago. He looks alright, but once you start to listen to what he says you can hear the lobbyists who pay his bills in every syllable. He is corrupt, foolish, and does not care what is good for the American people. So he's a pretty bad running mate.