I do see where people are coming from regarding Tomalak's portrayal in Plagues of Night / Raise the Dawn
. However, I personally view it not so much as Tomalak being written as foolish or a "dullard", and more that he's being portrayed as increasingly out of his depth. He ascended to the government through sheer ambition in a climate of great instability, as well as coming up through the fleet rather than through the Senate. When things settled down again, I think he was slightly lost - because under Kamemor the traits that got him to the top were no longer in vogue and he never truly cultivated the skills a successful politician needs. As a Romulan noble and fleet commander, who earns respect through a combination of birth status and long, patriotic service, he's a great success, but as a politician he can't really cut it. Romulan society is entering an era of change as of the end of Rough Beasts of Empire
, and for all his skill at riding the currents before then he can't really carve himself a role independent of Tal'aura. He just can't adapt.
In Vulcan's Soul: Epiphany
, which retroactively sets up Tomalak's interest in political matters for Death In Winter
, Spock notes that previously Tomalak showed little sign of seeking to acquire political leverage or make a name for himself outside the military. Indeed, it's suggested by Spock in that novel that Tomalak is taking quite a risk, that he's seizing the opportunity but possibly overplaying his hand. That seems quite consistent with what we see in later novels: he's a career military man, not a natural politician. His main talent is in knowing when to make his move and when to be reliable, a sort of sly cunning in how he plays his hand combined with aggressive, bold ambition when the time is right. That and a generally rather small-minded outlook aren't mutually exclusive.
He's later named Proconsul in the chaotic post-Nemesis
period after commanding the loyalist Romulan fleets protecting Tal'aura's regime. That is, by playing his hand at the right time and taking the reward, not through any natural affinity for politics (other than to the extent that such behaviour shows in and of itself an affinity for politics; but that's partly the point, that style of politics isn't Kamemor's way). Indeed, in Taking Wing
, as in Epiphany
, he comes across as a bit of a diplomatic and political amateur; he spends most of his time in that novel posturing or losing his cool, and has to be reigned in by Tal'aura.
Tal'aura and Tomalak worked quite well together, regardless, no doubt because they had similar views regarding certain issues; for instance, similar distrust of the Federation. He leaves office briefly when she dies, but returns to be co-Proconsul under Kamemor, who's a very different sort of leader. And he never really fits in, because without Tal'aura he's just a hot-blooded, suspicious, scheming reactionary, not a political player. As I see it, Tomalak doesn't understand a Romulan Empire that isn't rooted in the same aristocratic values and green-blooded posturing that he himself has embodied for decades; this new, populist leader he can't really understand. And now that he's pretty much at the top, and his ambition and service can't really take him any further, he doesn't have what it takes to shine. He's obsolete.
EDIT: I think also that for those of us familiar with Andreas Katsulas on Babylon Five
, our sense of the Tomalak character is sometimes distorted by awareness of G'Kar. It can't help but feel...wrong...to have him come across as less than insightful...