Is mercerized cotton less absorbent than cotton?

No. Mercerized cotton is more absorbant. Mercerising results in the swelling of the cell wall of the cotton fibre. This increases the surface area and reflectance, giving the fiber a softer feel and, if the fiber is held under tension, producing a shiny surface. Due to the increased surface area, mercerised cotton will absorb 7-8% more than unmercerised cotton. The process was originally developed to make cotton more readily absorb dye.