No it is not mandatory to have full member rank to join signature campaign. Atleast you need to have a Member rank. Although being a Full Member gives you a lot.of choice. If you want to participate in signature campaign first concentrate on ranking up by creating good topics on the forum.
Yes it is not mandatory but most of the paid signature campaign requires full member rank and only limited signature campaigns that accepts member rank which is the lowest rank being accepted. Although not all signature campaign pay weekly or per 2 weeks as there are also signature campaigns from bounty campaigns/ICOs.