"For there is one God, and one mediator between God and men, the man Christ Jesus" -- 1 Timothy 2:5
There is not "one God" among several gods. There is not "one mediator" among several mediators.
One God. One mediator, Jesus Christ.
This is incorrect even for a bumper sticker. Buit it is also irrelevant since Our Lady is not a mediator apart from Christ but an intercessor and co-redemptrix between those who choose to pray to her and Christ. Marian devotion is is no way bypassing Christ.
This is why it is also wrong: Christ is not the mediator between us and God, but quite simply, God. We are encouraged to pray to all Persons of the holy Trinity directly, and either Person receives the prayer without any need for Christ to mediate it.