My semen does not stiffen the fabric.should I worry? Is it a fertility problem?

Watery semen happens sometimes.

No fertility test is needed.

No don't worry. You are probably not letting it dry long enough. But it sounds as if you are concerned about your overall penis health... this is good actually because a lot of things can go wrong down there. What you want to use is a penis health creme called Man1 Man Oil. It is a multivitamin creme made just for the penis and will ensure that it is getting the vitamins and nutrients it needs to stay extra healthy and strong. I use it and it works very, very well. You can get it online. Cheers.

Be worried plenty that something so trivial worries you.

your worried and stressed over nothing

Sperm isn't responsible for the viscosity of semen.

But, yes - you should worry. Normal people don't try to judge their fertility by jizzing on their clothes.