The practice of placing crowns on images of the Virgin and Child arose (as I understand it) from entirely within Roman Catholic tradition, as a means of expressing a greater magnitude of piety towards the subject. A similar phenomenon exists in the East of placing a riza or oklad, an elaborately-tooled cover of silver or gold (often also studded with precious stones or pearls) over an icon, with often only the hands and faces showing through. This practice goes against the purpose of iconography, which, through its content, pictorial composition and use of color, expresses and proclains the truths of the Orthodox faith. Obscuring the icon in any way, however honorable and pious the intent, reduces the icon's ability to be the "window to heaven" that it should be.
I might also add that, while not all versions of Our Lady of Perpetual Help are "crowned", there is another, more important problem with this image: I have yet to come across any version, be it the "original" 15th century form, or a copy of it, which shows three stars of perpetual virginity on the Virgin's mantle. Sure, the star on her left shoulder would be obscured by the Christ-child, but there is no star at all on her right shoulder. There is one star only, above her forehead. This single star is characteristic of Roman Catholic Marian art.
I am in no way suggesting that the RC church does not uphold the doctrine of the perpetual virginity of the Mother of God, but this deficiency of doctrinal expression, though subtle and easily-missed, is an example of the legacy of the western church not taking up many of the canons and resolutions of the Quinisext and Seventh Ecumenical Councils, notably those on iconography. An excerpt from Leonid Ouspensky's Theology of the Icon:
The teaching of the Church on the christological basis of the icon, therefore, remained foreign to Western Christianity. This teaching could not enrich the sacred art of the West, which even today retains certain purely symbolic representations such as the lamb. The refusal to accept the decisions of the Quinisext Council later had, in the realm of sacred art, a great importance. The Roman Church excluded itself from the process of a development of an artistic and spiritual language, a process in which all the rest of the Church took an active part, with the Church of Constantinople providentially becoming the leader. The West remained outside of this development.
The Orthodox Church, on the contrary, in accordance with the Quinisext Council, continued to refine its art in form and in contents, an art which conveys, through images and material forms, the revelation of the divine world, giving us a key to approach, contemplate and understand it. It seems to us that it is particularly important for Western Orthodoxy, as it emerges in our own time, to be well aware of the significance of Canon 82 of the Quinisext Council. The canon, in fact, is the theoretical basis of liturgical art. Whatever course Western Orthodox art will take in the future, it will not be able to bypass the basic directive which was formulated for the first time in this canon: the transmission of historical reality and the revealed divine truth, expressed in certain forms which correspond to the spiritual experience of the Church.