Yes, voters believe in negative political ads, because research has proven that they work. The researchers that work on voting and elections have done studies. Those studies show that negative ads against opponents have more effect than positive ads for their own candidate. Voters believe bad things, and pay more attention to negative ads.
I think that voters do believe in negative political ads, it is true that most of the negative political ads are true, and it affects the decision and outcome of the political race in so many cases, so yes voters do believe in the negative political ads that they see.