Is Niacinamide as easily absorbed by the skin as Nicomenthyl 20?

No. Being Niacinamide hydrosoluble, its dermal penetration is hindered by the skin lipid bilayer (hydrophobic) and is not effective enough because it barely permeates the stratum corneum layer of the skin (Sara et al., 2008). From in vivo studies*, the Niacinamide’s absorption rate in 24h is only 2.2% of the initially applied dose, i.e. a very poor amount if compared to the actual quantity of Vitamin B3 released, in 24 hours, through the skin, by Menthyl nicotinate: 18% of the applied dose.
This means that Nicomenthyl 20 delivers 8 times more Vit. B3 than does Niacinamide, as per the graph shown below.

*Absorption of some organic compounds through the skin in man, by Robert J. Feldmann and Howard J. Maibach - The Journal of Investigative Dermatology – © 1970 The Williams & Wilkins Co.

Graphic Nicotinamide Nicomenthyl Skin Penetration