Does the Catholic Church accept that Mass is not an “essential service”?
The short answer is somewhat ambiguous here: yes and no!
Even your question is ambiguous: Does the Catholic Church accept that Mass is not an “essential service”? versus Does the Catholic Church accept that in-person Mass is “non-essential”? Which is it?
No as the morally established authority dictates, but yes in the sense of what the Church teaches, in respect to it’s faith and teaching about the the Eucharist. The obligation to attend Mass may be lifted by proper and duly instituted ecclesiastical authority, in times of necessity! Priests however must still say Mass, regardless if they have a congregation or not.
Civil Authority is the moral power of command, supported (when need be) by physical coercion, which the State exercises over its members. - Catholic Encyclopedia
I strongly doubt that the lockdowns concerning this particular pandemic constitutes an act of tyranny. If someone can prove that, please let me know?
However the Catholic Church is under no obligation to accept that the Mass should not viewed as an “essential service” as defined by governments!
However the legally established government can declare it to be a non-essential service. One has to remember the practicing Catholics remains a minority in comparison to the general population. Alas we are out numbered! The government must deal with the safety of everyone, Catholic or otherwise.
Even St. Paul states that we should be submissive to the governing authority.
Submission to Governing Authorities
13 Let every soul be subject to higher powers: for there is no power but from God: and those that are, are ordained of God.
2 Therefore he that resisteth the power, resisteth the ordinance of God. And they that resist, purchase to themselves damnation.
3 For princes are not a terror to the good work, but to the evil. Wilt thou then not be afraid of the power? Do that which is good: and thou shalt have praise from the same.
4 For he is God's minister to thee, for good. But if thou do that which is evil, fear: for he beareth not the sword in vain. For he is God's minister: an avenger to execute wrath upon him that doth evil.
5 Wherefore be subject of necessity, not only for wrath, but also for conscience' sake.
6 For therefore also you pay tribute. For they are the ministers of God, serving unto this purpose.
7 Render therefore to all men their dues. Tribute, to whom tribute is due: custom, to whom custom: fear, to whom fear: honour, to whom honour. - Romans 13:1-7
Even St. Peter the Prince of the Apostles and first pope tells us to be submissive to earthly authority.
13 Be ye subject therefore to every human creature for God's sake: whether it be to the king as excelling;
14 Or to governors as sent by him for the punishment of evildoers, and for the praise of the good:
15 For so is the will of God, that by doing well you may put to silence the ignorance of foolish men:
16 As free, and not as making liberty a cloak for malice, but as the servants of God.
17 Honour all men. Love the brotherhood. Fear God. Honour the king. - 1 Peter 2:13-17
Pope Francis urges for following COVID-19 lockdown amid church-state debate
Rome: Pope Francis waded into the church-state debate about virus-imposed lockdowns of religious services, calling on Tuesday for "prudence and obedience" to government protocols to prevent infections from surging again.
Francis' appeal came just two days after Italian bishops bitterly complained that the Italian government offered no provisions for Masses to resume in its plan to reopen Italian business, social and sporting life starting May 4.
While it wasn't clear if Francis intended to send a different message than the bishops, his appeal for obedience and prudence was in line with his previous calls to protect the most vulnerable, and for economic interests to take a back seat to shows of solidarity.
At the same time, Francis has certainly chafed at the lockdown, saying early on that he felt like he was in a "cage" and lamenting more recently that the church isn't really "Church" without a community of faithful present and the administration of sacraments.
Italian Premier Giuseppe Conte's government announced on Sunday that funerals could resume starting May 4, but there was no information on when the faithful could attend Mass. In response, the Italian bishops' conference expressed outrage that its proposals of safety protocols had apparently been ignored.
The bishops said that they "cannot accept that freedom of worship is compromised". They argued the government should have distinguished between its duty to provide health guidance and the church's right to "organize the life of the Christian community, respecting the measures but in full autonomy".
Conte's office hastily responded that it was working on protocols to allow the resumption of liturgical services as soon as possible but "in conditions of maximum security". Francis weighed in on the fraught issue Tuesday at the start of morning Mass celebrated alone in the chapel of the Vatican hotel where he lives.
The faithful would greatly rethink their opposition to government health rules if the Coronavirus produced symptoms similar to the Black Plague!
That said, the Catholic faith holds that the Sacred Mass is indeed an essential service, at least in the point of view that concerns the faith and the salvation of the world.
Personal attendance at Mass has been decreed as an obligation for Sundays, however the Church can lift this obligation for very serious reasons.
A Sunday obligation for the faithful to attend mass did not become an obligation until the fourth century.
Priests are still obliged to say Mass even without a congregation present. There are still extraordinary graces and benefits of a Mass without a congregation!

In Defense of Side Altars
Pope St. Paul VI called the Mass the apex of our faith. If the Holy Sacrifice of the Mass is apex of our faith, it would seems logical that in-person mass attendance would be considered an essential service! The Church as of recent, not endorsed or refuted this modern concept as yet. Perhaps one day She will. The term (essential service) is a modern day usage which the Church does not employ as yet!
Perhaps the following articles may be of interest here:
Addendum: Catholics within the Archdiocese of Vancouver are morally obliged to go to Church on Sundays as of July 10, 2021. The provincial health officer declared the “state of emergency” due to Covid-19 over.
The Coronavirus is a communicable disease. Could governments acted otherwise? Sure! But would the faithful follow other rules put in place? Not all, that much, I am positively sure of!