I don't know that they were trying to portray him as a simpleton, exactly - that might be overstating it. I'd say he's been characterised more as reliable, straightforward, solid, dependable, but without that extra spark that makes a person a legend in the galaxy. Perhaps a Captain deSoto compared to a Picard - someone who's perfectly good at running a starship and being a captain to his crew but not one of the standouts.
Perhaps in-story the issue comes from other characters thinking that Tomalak thinks he's better than they think he is... if you followed that. He keeps trying to put himself in higher positions of authority, thinking he is
one of the special ones and he should
be a star on the galactic scene, but the others recognise that he's just not all that special after all, which is why he keeps getting his overly ambitious plans foiled.
That's how I saw it anyway.
I can't speak for older novels, I don't rememebr him appearing in any that I read.