It would depend on why the person wasn't paying attention.
Sometimes we simply have trouble concentrating due to the time of day, how tired we are, what's happening in our lives, etc. Such wandering of attention is not necessarily sinful, let alone a mortal sin.
However, if a person was actually disdainful of the Mass and the sacrifice of the Eucharist, then it would be sinful and could possibly venture into the territory of mortal sin. In such a situation, it's not the simple act of not paying attention that is sinful but rather the disdain for the sacred actions taking place.
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