If our submarine radars can’t detect underwater mountains, how good can they be at detecting enemy subs?
LOL. Radar doesn’t work underwater.
As another poster pointed out, submarines do not use radar--at least not underwater that is. (They do use radar when surfaced to avoid traffic.)
But to your point about not detecting mountains, here's the thing: submarines use passive sonar to detect other submarines. If they used active sonar, they could be detected by the submarines they're hunting well before they'd get a return ping from their active sonar.
But by using passive sonar, it makes silent underwater mountains undetectable. Best to mind your nautical charts!