I would guess, that MM is a very minor factor for TLC. At least the precision of TLC may not be enough to separate that. I would argue that difference in MM compare to the phenomena that concentration of the solute in the column of solvent should be different with height. And this effect does exist if I remember correctly in the ocean where NaCl concentration depends on the depth, but it is pretty large column of water. In TLC they do not make such large size and capillary effect will not have enough force to operate with such huge height anyway.