I don't think helmets are worn for aerodynamics. IIRC professional race cyclists only started wearing helmets after in 2003 when they became mandatory. If there had been any advantage in aerodynamics the athletes would have voluntarily picked up helmets before.

Also, anecdotally, I am occasionally road cycling for sports and do wear a helmet at all times. However, I am certainly not doing that because of aerodynamics.

Edit for clarity.