Is it worthwhile to apply seamsealer to a nylon skirt and also to waterproof it with tent waterproofing?
yes/no
If it is new you shouldn’t need to seal but if several years old go ahead and seal away. But I am aware that some off shore box store skirts may use coated waterproof fabric yet not take the step of sealing the seams. Buy USA made skirts.
No, seams aren’t the problem
Most nylon sprayskirts are waterproofed. It is the nature of the beast that they let water in at the coaming and through the top of the tunnel. If you really want watertightness, get an all-neoprene skirt.