Of course if the signature campaign requires many number of posts to be satisfied your posts will decrease in quality. But this depends by you not by the fact you have a signature or not. I do not look at the signature of a member if I wanna give him a merit or not.
I do have a signature campaign, and it requires quite a number of post to be counted - to have payment for that span. But I also keep my posts as constructive as I can, meaning even if there is a number of post required I will not sacrifice the quality of my post just to reach the quota.
The signature has nothing to do with you gaining Merit.
My suggestion could be, in case, to join signature campaigns that requires less number of posts so in this case you can post messages with more quality, without caring at the number of messages you have to do in that week.
As I've said, that's not the case. The number of post count required for a campaign should not affect the quality of your post. In fact you need to really create constructive posts as you will have the chance of gaining Merit(s).