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.
Exactly, signature campaign is just a way to promote the project and to spread awareness on the crypto community.
The credibility relies on the project itself. If the team are true to themselves and serious to achieve their goal. Then they will check every aspect to make their project succeed specially for investors to trust them.