It is not a very energetic event for its size. The iceberg is floating, it can't accumulate a lot of potential energy before flipping, so when it happens, there is not enough to make big waves. The center of gravity of the iceberg barely moves.

A mountain sliding into the sea is different, these higher up rocks have a lot of potential energy, that convert into kinetic energy as they fall, which is transferred to the water as they splash down, making big waves.