I always felt given the incompetance of the Romulans that they were more a comical disturbance rather than a genuine threat that they were often trying to portray themselves as. This was rather evident with Tomolak. Whereas other Romulans were genuinely serious, Tomolak fit the trope of a bad liar, a very comical usage of the trope. His moments in "The Enemy" in which Picard rejects his claim, are hilarious as Hell.
There's no indication that Tomalak is, or is intended to be presented as, a bad liar. If he's not especially convincing, it's because his claims are rather unbelievable on their face - but this is politics, where what matters isn't what you believe, but what you can prove. A bland lie, even if it's obvious, is useful because it doesn't give anything away.
Consider for a moment the ambasador in, e.g., The Hunt for Red October. Did the Americans really believe that the Soviets had simply "lost" their most advanced nuclear submarine? No, of course not. Were they meant to? Probably not, or a more credible lie could be drawn up. But it wasn't because it didn't matter: they couldn't prove anything else in the eyes of the world.
So it is here. Tomalak says that the expedition to Galorndon Core (in 3x07 "The Enemy") was a navigation error and that the satellite in orbit of Nelvana Three (in 3x10 "The Defector") was for archaeological purposes. Approximately no one believes this, but it doesn't matter because they cannot prove otherwise. Tomalak doesn't need anyone to believe his explanations, he just needs to give them for form's sake.
Proof and evidence are an important political consideration in both episodes:
WORF: Captain, I see no reason to return the Romulan to his ship. He should be held and interrogated.
RIKER: We have every right to detain him, sir.
PICARD: Without evidence of intent, Number One, it will not be a simple matter.
HADEN [on monitor]: Captain, we have received an official protest from the Romulan Empire demanding the return of your defector. Obviously, we are refusing to comply. I join in your skepticism, but if it is a deception, the Romulans are certainly making a good show of it.
Beyond the characterization of Tomalak, there's every sign that the Romulans are to be taken seriously in "The Enemy":
PICARD: Commander, both our ships are ready to fight. We have two extremely powerful and destructive arsenals at our command. Our next actions will have serious repercussions.
RIKER: Close call.
PICARD: Too close, Number One. Brinksmanship is a dangerous game.
And in "The Defector":
PICARD: If the cause is just and honourable, they are prepared to give their lives. Are you prepared to die today, Tomalak?
TOMALAK [on viewscreen]: I expected more from you than an idle threat, Picard.
PICARD: Then you shall have it. Mister Worf. [Picard signals Worf, and three Klingon warships decloak around the standoff.]
PICARD: What shall it be, Tomalak?
TOMALAK [on viewscreen]: You will still not survive our assault.
PICARD: You will not survive ours. Shall we die together?
Moreover, the Romulans in general were treated as very threatening adversaries. In addition to threatening to destroy the Enterprise at least six times ("The Enemy", "The Defector", 4x11 "Data's Day", 5x24 "The Next Phase", 6x14 "Face of the Enemy", and 6x25 "Timescape", albeit the last by accident) they also conspired at least twice to throw a wrench in Klingon-Federation relations (4x26 and 5x1 "Redemption" and 4x24 "The Mind's Eye") and tried to invade Vulcan (5x07 and 08, "Unification").
The Romulans' first appearance was in the TOS Episode Balance of Terror, which was a kind of remake of a WWII submarine movie (The Enemy Below).
In TOS, the Romulans were definitely not a comic relief, they were serious (but honorable) adversaries. The Klingons, however, were the sneaky, back-stabbing guys. Between TOS and TNG these two races swapped their characteristics, turning the Klingons into the honorable and the Romulans into the sneaky guys.
So, if at all, the original Klingons might have been invented as a kind of comic relief, but not the original Romulans.