Mortal sin (and you can find more about this in answers to other questions) has several aspects: personal freedom, grievous (serious/major) matter (the thing itself) and clear intention. I would say in this instance that the matter is not serious enough (unless the context--in Church or some place where it would be particularly out of place--is especially free from such words). Profanity, I'm sorry to say, is rampant in our society, so the use of vulgarities hardly causes a ripple in many groups. So, not mortal sin.

It's better not to encourage the habit of profanity, though, because people who do are guarded at first, but become careless--and use worse language yet or use vulgar or profane language in places where it's especially inappropriate and shocking to others.