If all the matter in the universe eventually fell into a black hole, would time stop?
I don’t think you could say that… from the point of view of anything outside a black hole, time is frozen for objects passing the event horizon. But time is still normal for anything outside the black hole. (If there’s nothing outside maybe it’s not meaningful to talk about “anything outside the black hole”.) Now if we consider matter inside the event horizon (which as far as we know, would get squished to oblivion, but leaving that aside for the moment- possibly for very large black holes with large event horizons, there’s a fair-sized region for which normal matter wouldn’t be destroyed), time for that matter inside seems to run normally; it’s the outside time that’s speeded up. So, there is always somewhere where time runs normally.
