In my opinion, missing Mass on occasion cannot be a mortal sin, even if it is done without a serious reason. Mortal sins are those sins that are entirely incompatible with the state of grace, with the infused theological virtues of love, faith, hope, with the indwelling of the Holy Spirit. A mortal sin must violate a positive or negative precept, i.e. the thou shalts and thou shalt nots of the moral law.
Missing Mass on occasion for a just reason (not necessarily a grave reason) does not, in and of itself, violate the commandment to worship God and to keep holy the Sabbath. Both of these things can be done apart from the Mass. Also, omitting the worship of God or the keeping of the Sabbath from time to time does not necessarily violate the positive precepts, since these are not continually binding. One cannot continually, day and night, fulfill every positive precept in every way. So the commandments to worship God and to keep the Sabbath holy are not violated, at least not in any serious way, if a Catholic occasionally misses a Mass on the Sabbath or a holy day.
Also, the moral law applies to everyone, not only to Catholics. One would have to conclude that every non-Catholic, even devout Protestants, in the world is committing objective mortal sins every time they omit attending Catholic Mass on a Sunday or holy day.
There is a group of nuns in Australia who went out to minister to the aboriginal peoples there. They were ordered by the Bishop not to go out to minister unless a priest was with them, lest they miss Mass on a Sunday or holy day. They went out anyway, and did miss numerous Masses. They did not have to go out to minister; they could have ministered 6 days a week and gone back to a city to attend Mass. So this might not qualify as a grave or serious reason. They did the right thing, because they were imitating Jesus, and fulfilling the commandments to love God and neighbor.
Missing Mass when one is ill with a cold or fever or other relatively minor ailment is not a mortal sin. Yet this is not a grave reason, but only a just reason. Missing Mass because bad weather makes the driving somewhat more dangerous, but not very dangerous, is a just reason, but not a grave reason. No one claims it is a mortal sin to miss Mass for these reasons.
Thus missing Mass, on occasion, for a just reason is moral. One need not have a grave or serious reason. Just reasons would include taking a vacation or going on an excursion (hike, etc.) where Mass would not be available, spending time at a family gathering, traveling extensively for business, working at a job where the hours occasionally make it difficult to get to Mass.
If someone were to miss Mass for no real reason, just or grave, I conclude that this would be a venial sin. It is not incompatible with the state of grace to decide not to go to Mass on a holy day of obligation, or to decide to miss Mass once in a while on a Sunday. It is a sin to do so without a just reason, but not a mortal sin.