Missing mass is not a mortal sin unless all the conditions for mortal sin are present. Is it a grave matter? Yes, objectively, see the citation above. We are commanded to worship God by the 3rd commandment, we are commanded to participate in the Eucharist by Jesus Christ. It is the source and summit of all Christian life. Does the person know it is wrong? Yes, you do now. Previously you may not have been properly instructed. You naturally do not rely on strangers on a forum for your spiritual direction, so consult the experts on CA home page, the Catchism, the gospels, and of course your own confessor. Is the action or omission committed willingly and deliberately with intent and full freedom to act, without a compelling reason. Your own good judgement and conscience can decide what is a good reason to miss Mass–illness, care of child, elderly or ill person, act of charity for someone, mandatory work etc. Naturally if there is a reasonable possibility of attending Mass at another time and place that weekend you seek it out. Is missing Mass is a deliberate choice especially for reasons that flout the reasons for attending Mass: I don’t want to be around “those people”, I don’t need to to worship as the Church tells me to, I can worship my own way (or not), I do not choose to worship God at this time ordained for His worship, I habitually disregard Church teaching on this and other subjects, I am spiritually lazy (sin of sloth) etc.
If those 3 conditions are present it is a mortal sin. The more it becomes a habit the greater the weight of the sin and the more damaging to the soul, and the less likely it is the person will seek healing in confession and the other sacraments.