Change your preferences here.
Deals damage and has a 100% chance to lower the target's Speed by one stage.
For the most part, Mud Shot should not be used competitively since it is weak. However, the secondary effect to lower the target's Speed is desirable for some Pokemon to turn a potential check switching in into a non-threat. Other than this, however, one should always use Earth Power.
Earth Power is a more powerful option.
Icy Wind is an Ice-type equivalent.