One answer to this question is that a motorcycle helmet DOES require a certain amount of thickness to meet the DOT ratings. Whether this makes it ‘uncool’ visually, is more of a personal opinion.
According to the National Highway Traffic Safety Administration NHTSA, 14 percent of riders use helmets that do not comply with the DOT rating — defined in the Federal Motor Vehicle Safety Standard (FMVSS) No. 218.
These non-conforming helmets are often ‘novelty’ type helmets made for fashion over function.
For me, there’s no way I would ride my motor bike with a helmet that does not meet the DOT standards. But, many riders choose to ride without a helmet.
Testing In 2006 the NHTSA performed there standard impact test on 9 novelty helmets andzero of them passed. Actually, none were even close. For the impact attenuation portion of the test , the average novelty helmet imparted 989g on the test subject. The minimum allowed to get DOT approval is 400g. In addition, many of the novelty helmet straps broke during the test… under relatively light loads [132 pounds vs. the required 300 pounds].
DOT Motorcycle Helmets - Novelty Motorcycle Helmets