look, there are exaggerators in all religions.
This is not Catholic dogma. It’s an exaggeration of the term “mediatrix of all graces”. But this merely means Mary mediates graces, but so does every saint and every Christian.
When I treat a patient, I am the “mediator” of God’s healing (as Pare said: I treated him but God cured him). And when we pray to Mary or the saints, or ask for our friends to pray for us, they are “mediators” of the graces that God sends to us (i.e. the middle men who help us gain God’s graces).
When Jesus said “when two or more are gathered in my name I am there” he was pointing out the importance of joint prayers. No, it’s not necessary, but it is part of our belief.