One thing to keep in mind is that we're dealing with four separate legal systems - Fereldan, Orlais, The Free Marches (which is a collection of city-states all with their own customs) and the Circle itself. But as far as I can tell, there are no laws explicitly forbidding mages from owning property or holding titles. Just social custom, and the rules the Circle imposes.
It's established that mages can leave the Circle with special permission from the First Enchanter, so it's possible that a mage could be granted leave to return home and take up their mantle of nobility.
But the flip side: magic is viewed with suspicion and distrust by the common folk; so it's not likely that the people would be okay with being ruled by a mage. Especially because the Southern Chantry teaches that mages are not meant to rule. Having a mage noble would smack too much of Tevinter custom.
Lastly, it's highly unlikely that a mage noble would be able to continue their line. Magic has some element of heritability, and it's not likely that a noble family would consent to have magic added to their bloodline. Leandra, Hawke's mother, is disinherited when she marries a mage for just that reason, and is reduced to living like a peasant in Fereldan. So a mage noble is probably not going to be able to marry & have children; one of the main requirements of the nobility. It seems to me that most noble families, once the heir-apparent manifests magic, would rather just find a new heir (apostate lords aside). I imagine naming a mage a noble heir would be a last-resort option; if there was no one else who could possibly inherit.
So, a mage could potentially be a noble, but a lot of things would have to fall into place for that to happen.