Did Tim Kaine cost Hillary votes

[Some of My Best Friends Are Gay]

Likely. Maybe Kaine’s past opposition to abortion as Governor of Virginia, for example, may have led some pro-choice voters to stay home on Election Day.

I would doubt this.

It's more likely that his centrism on economic issues led some progressive voters to stay home or cast a protest vote for a third party, but whether this actually had an impact on the election as a whole is very hard to say, and I'd guess it didn't really.