Wearing diapers in particular may not cause low sperm count, but wearing thick or tight-fitting undergarments (of any sort) can raise the temperature of your testicles, which has been shown to decrease sperm production. Comparing how well cloth vs. disposable diapers "breathe" is a good thing to look into. Keeping cool and well ventilated could help improve your sperm count.

As far as the shape of sperm goes, normal sperm have oval-shaped heads and long smooth tails that move them. Sperm with heads that are too large or small, or tails that are curled or kinked, don't have as much ability to reach and fertilize an egg. It's the quality, not the quantity, of sperm that's most important. Sperm must have few abnormalities and move quickly and easily.   - MG