No, signature campaign is just part of the marketing strategy. Signature campaign is being used to promote and increase the reach of the project, but it does not increase the credibility. It always depends on how promising the project is, over all including the idea and so on.
Credibility of the ICO is definitely based on the project itself but sometimes having a sucessful signature campaign can also create good impression on the project particularly if the bounty manager has carried the bounty project well. The credibility of ICO nowadays is not that high anymore compared to its previous years but still some good projects still managed to survive because of its legitimacy.