She might be. It's a ship of 80 (now) to 180 (S1 & 2) crew, so a Lt j.g. might be enough to run the medical department. DS9 had about 300 residents in the early seasons (probably many more as it went on), and the CMO was a Lt j.g. straight out of the Academy.
The one issue, of course, is rank. If the CMO is a Lt. j.g., then why do we see one Commander (METMTSMGM) and a Lieutenant Commander (Culber) working on her staff? The answer is likely specialties. If the Commander was a specialist in xenobiology, or something, he might not be directly under her. Culber, of course, works in sickbay, and at one point assists the CMO in a surgical operation. But, we don't know his exact title or position. Could Pollard be in charge of the medical administration onboard the ship, but Culber just be the chief of surgery? Could he have transferred from a higher position on another ship to be with his husband on the front lines of the War, despite his rank?
Maybe. I think the medical staff might not pay as much heed to rank as the other departments (specifically security and command), as they likely take into account experience and talent outside of Starfleet.
Regarding rank for the CMO position, it should be noted that when Bashir was first assigned to DS9, it was essentially a backwater installation with a minimal Starfleet presence, so a Lt. j.g. is not only possible, I find it likely. This would be his first assignment for his career, get good experience, and move on to a bigger assignment. Then the wormhole was discovered, and it got bigger credibility and became essential as the Dominion situation escalated.
Discovery, at the time it was launched, was pretty state of the art, so having a Lt. j.g. as CMO is unlikely. Especially with the high amount of Commanders and Lt. Commanders onboard. DS9, at first, really only had two Starfleet officers higher ranked than Bashir... Sisko and Dax. I'm sure there were a few other full Lts. around in the background, but not many. Even in later seasons, and Bashir became a full Lt., we only saw a handful of higher ranks assigned there... Sisko, Dax, Worf, and Eddington. Even during the War, it didn't seem like many higher ones were assigned there, and the few we saw were likely from passing ships.