We need to distinguish between the author creating the Ashmounts intentionally and TLR creating them intentionally.
Clearly, the author put them in there for a reason, since authors don't generally add details without a purpose. Sanderson's purpose could just be the dismal setting, but (as has been discussed) the proper names may be a clue that there's more to them than that. Another clue is that "ash" is the very first word in the first book. I don't know what this purpose may turn out to be, though. I haven't seen any evidence yet that the ash is sentient or responsive to certain people or magics, the way the mists seem to be. Nor do I know of any correlation between heavy and light ashfalls and the other events.
As for TLR, I agree with those who said the Ashmounts were just an unintended side effect of his re-making the landscape. He clearly didn't love the ash that much - as I recall, he kept Kredik Shaw and his own clothing quite clean, and also had the skaa sweep the ash out of Luthadel.