My take on this is a little different than Bob's.
In your question, you did not make any distinction between being a missionary and being a Catholic missionary.
No Catholic missionary can take their vocation seriously if they don't renew their Sunday Covenant by going to Mass.
At Mass (accompanied by regular Confession) we are strengthening and fortifying our souls with the sacramental graces we need to fulfill our vocation.
While being a non-denominational missionary is an admiral vocation, those in this endeavor are only spreading part of the Gospel and not he whole Gospel as Catholic missionaries do. They also lack the sacraments of the Church or at least most of them.
I would take Bob's advice but if this turns into a long-term, serious vocation, you want to ensure that wherever you preach the Gospel, faithful Catholics and the sacraments of the Church accompany you. In this sense, your mom is right.
Just my two cents.