Basically Marian devotion can seem excessive to Protestants because they ignore Mary so much.
Mary has been whitewashed out of much Protestant spirituality, so that for people who have only known that, it is a shock to see her in a prominent place in the church. They are used to spending much of their time on Paul and his writings, but none on Mary. She is often presented as a transient figure who shows up briefly at the nativity and then is never mentioned - as someone said - like an embarassing mother-in-law.
But the historic church has paid great attention to Mary since the earliest times.
In fact there are so many things that are unique to the Virgin Mary.
She is the Woman Propecied in Genesis 3, Isaiah 7, and Revelation 12, whose seed crushes the head of the serpent.
She is the woman uniquely to counter the disobedience of Eve, by saying Yes to God, rather than No, as Eve did.
She was the only one who gave her flesh and humanity to
the Son of God, and she is uniquely to be blessed throughout all
generations (Luke 1:48).
She is the only human in scripture to be named Kecharitomene, that is, Full of Grace.
Mary was the first human being who did receive Christ. Out of the millions of “decisions” made for Christ, Mary’s was the first. Therefore, whatever promises the Holy Scriptures hold for us, Mary already possesses. If the sacred Scriptures declare that we are all kings (Revelation 1:6), is it so
strange that the Church refers to Mary as Queen?
She is the mother of the Son, therefore she is also the mother of the Church, which is Christ’s body.
If the Holy Bible promised that you and I shall judge angels (1 Corinthians 6:3), is it so odd that the Church should sing that Mary is “more honorable than the cherubim and more glorious beyond compare than the seraphim”?
If we who are called “holy brethren” (Hebrews 3:1) are commanded to be holy as God is holy (1 Peter 1:15, 16) and are to present our bodies as a living sacrifice (Romans 12:1), is it so unthinkable that she whose holy body was the recipient of God Incarnate should be called “most holy” by the Church?
Mary calls herself an handmaiden, a servant of God. She served God, God her Saviour. And these are precisely the qualities that God rewards with honour.
Ps 149:4
For the LORD takes pleasure in his people; he adorns the humble with victory
Jas 4:10
Humble yourselves before the Lord and he will exalt you
Matthew 5:3
3 "Blessed are the poor in spirit, for theirs is the kingdom of heaven. 4 "Blessed are those who mourn, for they shall be comforted. 5 "Blessed are the meek, for they shall inherit the earth.
Mark 9:35
And he sat down and called the twelve; and he said to them, “If any one would be first, he must be last of all and servant of all.”