I had the similar feeling that the OP did when I was reading the duology. I did think that DRGIII portrayed Tomalak as something of a dullard, and made sure to have other characters, namely the Praetor, refer to his lack of smarts more than once. I never got that impression from watching him on TNG.
Granted he was thwarted by Picard, but being the bad guy, he had to be. I rather wish DRGIII had just invented a character or killed Tomalak off instead of portraying him that way.