The Tatars as a people didn't really coalesce as a unit until after the time range of CK2. The Mongol invasion led to the Turco-Mongol fusion that resulted in the group known today as Tatars.
Instead, the Tatars are split into multiple ethnicities like the Bolghars and the Cumans. Basically, during this period, "Tatar" meant "anyone from the general region known as the Tartary" which we see in game as the Empire of Tartaria.
I suppose there could be a culture-flip situation where Turkic peoples owned by a Mongol ruler could flip to Tatar. That said, while much of the area remains Nomadic, you wouldn't see this very often.