Offering Mass alone is not “less of a Mass” but it is discouraged by Canon Law:
Can. 906 Except for a just and reasonable cause, a priest is not to celebrate the eucharistic sacrifice without the participation of at least some member of the faithful
Regarding the Traditional Mass
Council of Trent Session XXII
americancatholictruthsociety.com/docs/TRENT/trent22.htm
CHAPTER VI
THE MASS IN WHICH THE PRIEST ALONE COMMUNICATES
The holy council wishes indeed that at each mass the faithful who are present should communicate, not only in spiritual desire but also by the sacramental partaking of the Eucharist, that thereby they may derive from this most holy sacrifice a more abundant fruit; if, however, that is not always done, it does not on that account condemn as private and illicit those masses in which the priest alone communicates sacramentally, but rather
approves and commends them, since these masses also ought to be considered as truly common, partly because at them the people communicate spiritually and partly also because they are celebrated by a public minister of the Church, not for himself only but for all the faithful who belong to the body of Christ.
Canon 8. If anyone says that masses in which the priest alone communicates sacramentally are illicit and are therefore to be abrogated, let him be anathema