This is the context of the Luther quote you posted: "the honor given to the mother of God has been rooted so deeply into the hearts of men that no one wants to hear any opposition to this celebration. There is rather a desire to further elevate it and make it even greater.... I worry that we give her all too high an honor for she is accorded much more esteem than she should be given or than she accounted to herself... Christ is diminished by those who place their hearts more upon Mary"
Thank you for letting me know about this. I was unaware of this context; I found the quote on a (Catholic) site that spoke favourably of Luther’s Marian devotion.
If we place the full quote in its historical context it is clear Luther is commenting on the excesses of Marian devotion in his time. Every Catholic would agree that Christ should be our primary focus and that Mary’s role, as Mother of the faithful, is leading us to her Son. When it comes to this role, we could never accord her enough esteem!