They are mostly Humans.
SCAG describes the Uthgardt people in many places.
Quadratic Wizard's answer already tells us about the history in earlier editions, but the Sword Coast Adventurer's Guide also presents the history for us in 5e, and it's pretty consistent. Page 94 states, under the heading "The People of Uthgar":
The saga begins in the distant past when the humans of the North lived in fear and isolation. [...] Each of the defeated spirits became embodied in the totem of one of the groups of humans who followed and revered Uthgar.
Almost all of the original tribes of the Uthgardt are active in the North today. Even a tribe thought to be lost or destroyed might be represented somewhere by a small number of humans who claim to be descended from the one of the tribes of old, but such people, where they exist, aren't numerous by any means, and their claims are often spurious.
On pages 92 and 93, Aedyn Greymantle describes the Uthgardt Lands (under the heading of the same name). Unless I missed something, nowhere does it state a specific race, but it does state on page 94:
Young men and women looking to make a name for themselves sometimes build their reputations by hunting dangerous predators and great beasts
which indicates, at least, humanoids.
It also describes the friendship of Aedyn with Gyrt, the leader of one of the tribes. In particular, this excerpt from p. 93 might help.
Over the years, as I earned Gyrt’s respect and she mine, we became friends, and I came to know her three sons as well. Though Gyrt died some decades ago, I still visit her children, whom I played with when they were young. They now have children of their own, and all call me auntie.
This indicates lifespans of humans and similar races, as if the children that grew up were, for example, elves, it would take "centuries" for such a thing to occur. This is only about a specific family, though.
Uthgardt Tribe Member is a playable background, though
The Uthgardt Tribe Member background described on p. 153-154 does not specify any race restriction; as such, player characters of any race can have this background.