Let's take stock. Cora is dead. Uncas is dead. Magua is dead. Our faith in humanity is dead.
Jeepers, that's a tough ending. The bad guy is dead, but the young Mohican and his lovely potential wife are also dead. This is not the kind of finale that's going to leave you exhaling and saying "Well! I feel just dandy about life."
There are a couple of ways of reading this rough ending. You could conceivably read this as a condemnation of interracial love, given that the pure white couple gets to live happily ever after and the union of Native American Uncas and biracial Cora is never going to happen.
But you can also read in the bigger racial terms discussed in "What's Up with the Title?" Uncas was the only guy who was going to carry on the lineage and heritage of the Mohicans, and he gets cut down in his prime. Chingachgook is the only Mohican left, and he's put in this super lonely position because of white settlement of Mohican land.
Not only that, but Uncas is killed because he's acting in service of white settlers—he's trying to get Cora Munro back from being abducted by the baddie-bad-bad Magua. So white men are (indirectly) the reason he dies, and (pretty dang directly) the reason that the Mohican tribe ends.