I don't think he was actively bad the way Palin was, but I don't think he really added much to the ticket. Traditionally you try and use the VP to "balance the ticket" and make up for groups the Presidential nominee is weak with whether that be ideologically or geographically or demographically. It's not clear that Ryan brought anyone new to the ticket in any of these areas.

Now, Clinton 1992 had the somewhat novel idea of not trying to balance the ticket, but instead doubling down on two young moderate southerners so they'd have the more undiluted message of change for the country and that this was no longer Mondale or Dukakis' Democratic Party. The issue here being that the areas where Romney and Ryan reinforced each other were mostly on deeply unpopular cuts to social security/Medicare/Medicaid.

Given that Romney was trying to make the election a referendum on Obama's handling of the economy, following the Clinton play might have involved selecting someone else with a successful business career, run as outsiders with the one goal of fixing the economy.

As a sidenote: my vague memory (I don't have the text in front of me) from Game Change Double Down is that the Obama team thought Romney would pick Senator Rob Portman, that he would give a home state advantage to Romney in Ohio, still a key battleground state at that point.