Assuming that by mutual masturbation you mean stimulation of the genitals outside the context of the conjugal act, the answer is no. The reason is because it involves freely using the sexual faculty while actively frustrating its immediate end of getting semen into the vagina, as well as frustrating the faculties’ ultimate ends of procreation (which includes the rearing of children) and unitive love, thus making the act a perverted act.
Obviously, it frustrates the procreative end inasmuch as the activity is per se incapable of producing a child. But it also frustrates the unitive end inasmuch as unitive love depends on the biological union of male and female that strives for the end of begetting children. Without the “one flesh” union that is generative in nature, there can be no real spousal union.
The Catechism of the Catholic Church is unequivocal in its teaching on the immorality of masturbation:
By masturbation is to be understood the deliberate stimulation of the genital organs in order to derive sexual pleasure. “Both the magisterium of the Church, in the course of a constant tradition, and the moral sense of the faithful have been in no doubt and have firmly maintained that masturbation is an intrinsically and gravely disordered action.” “The deliberate use of the sexual faculty, for whatever reason, outside of marriage is essentially contrary to its purpose.” For here sexual pleasure is sought outside of “the sexual relationship which is demanded by the moral order and in which the total meaning of mutual self-giving and human procreation in the context of true love is achieved” (2352).
This is not to say, however, that stimulation of the genitals within the context of the conjugal act is immoral. In this case such stimulation is not considered masturbation. It is commonly held among moral theologians that such activity is morally permitted on condition that such activity does not pervert the procreative and unitive ends of the sexual faculty. In fact, such activity can enhance the achievement of the two ends, and thus be considered good.