They do change things between film and movie. Perhaps this version of Dormamu has isn’t her Uncle. Maybe he’s her father’s brother’s nephew’s cousin’s former roommate.

Yeah why not?

Magical ability to alter appearance of a human

if she is Dormammu's niece, it might be possible for at least one of her parents to be either human or humanoid.

Dormammu, in the comics, looks comparatively much more humanoid. But I mean, Clea doesn't have to be a human. Dormammu isn't, after all.

about not have to be human; she technically would be half of Dormammu's race and half of another.

Why would she be half of another though?

currently not sure whether MCU's version of Dormammu's race has actual humanoids; just Dormammu's head was shown.

In the MCU, it seems like Dormammu and the Dark Dimension are one and the same, like his very body is the entire dimension.

By the way his torso was shown. He punched Strange with a clenched fist.