Is it possible to make homemade crème fraiche using clotted buttermilk?
Chances are the "clots" are harmless. Give the buttermilk a sniff and a (small) taste. Does it seem OK? If so, the curdling probably just means that the cultures are still healthy and doing their job.

Whisk it in, and stay the course. Your crème fraiche should be fine.

Of course if the buttermilk smells or tastes spoiled, throw it out.

Buttermilk (and to a lesser degree, dairy in general) is kind of unique in the food safety world in that the sniff test is 99.9% accurate. If it smells OK and tastes OK, it almost certainly is OK.