It does not, though having 0 tone and 100 thickness will give the same results as if you had 20 in your belly stat. More specifically, you get a tempBelly modifier of (thickness - tone)/5, which is added to any points you have in the bellyRating stat. It is currently impossible to raise bellyRating itself.
You can also temporarily increase your belly rating with cumflation, but I can't figure out at a cursory glance what the exact modifiers are. Pregnancy will also increase the rating and tack on an extra line at the end.