There may be several reasons why a cat covers its owner’s mouth:

Playing behavior: The cat may regard this action as a game or interactive way.

Curiosity and exploration: They may be interested in their owner’s facial movements or sounds.

Attention-seeking: hoping to attract the attention of the owner.

Instinctive reactions: For example, when catching prey, they may cover it with their claws. However, the meaning of a cat's behavior may vary based on individual differences and specific situations.