This is the TRUE definition of mother bear. Mother bear is used as an exclamation to show the highest or lowest (depending on the context) possible level for something to achieve. In other words, it represents the epitome of any adjective. All you have to do is use it in a simile or a metaphor (metaphors are much more common). For example, if something is ANGRIER THAN A MOTHER BEAR, it is not possible to be any angrier than that, period. If it is incredibly cold outside, like absolute zero, then it is COLDER THAN A MOTHER BEAR. Get it?
John: It wasn't hilarious....it was funnier than a mother bear!