Are helicopters impacted by fog more than airplanes?
No.  If anything, given a helicopter can fly a much slower cruise speed than an airplane typically does, they do not require the visibility minimums which fixed wing aircraft do.  VFR weather minimums for a helicopter in uncontrolled airspace are 1/2 mile versus 1 mile for airplanes.  Be that as it may, a helicopter is still vulnerable to Loss of Control Inflight (LOCI) due to pilot error just like any other aircraft.