Is this an accepted Catholic belief [that Mary is the perfect creature]?
You must be careful when you say 'perfect.' If 'perfect' refers to the quantity of grace and holiness, then it may be said indeed this is Catholic belief. But that Mary is the 'perfect' creature may be more difficult, since the angelic nature is a much, much higher nature and far more glorious than the human nature.
However, we don't know how much fundamental nature (what it means to be, minimally, angel or human for example) and supernatural grace interact and work together to make a finaly 'perfect' creature as a whole. It could well be that the blessed Virgin Mary is the greatest creature because, although far below the ranks of the mighty angels by being human, could far exceed them in grace and honor—which one can definitely say is Catholic even if not dogma strictly speaking (Eastern Christianity, such as the Orthodox, would also affirm this uniqueness of the mother of God).
For example, Jesus Christ is (present tense) also a man. But He far exceeds the angels in dignity, grace, might, etc., per Hebrews 1, even though according to the same Scriptural testimony, He was made human—lower than the angels. So we see that indeed it can matter what or who you you are or what role you play or perogatives you enjoy in addition to being human that determines your actual greatness.
Traditionally for Catholics, being the mother of God Himself is both literally true (Mary is literally the mother of the Word made flesh, who is the same Word who existed before His birth; Jesus is literally Mary's Son) and obviously the greatest prerogative ever given to a creature. Making her the greatest creature in this better usage of the word 'greatest' or 'perfect.'
Pope Bl. Pius IX in his Bull Ineffabilis Deus (the document wherein for the first time it was defined officially and finally that Mary was conceived [specifically] without sin) wrote in its introductory paragraphs:
Therefore, far above all the angels and all the saints so wondrously did God endow her with the abundance of all heavenly gifts poured from the treasury of his divinity that this mother, ever absolutely free of all stain of sin, all fair and perfect, would possess that fullness of holy innocence and sanctity than which, under God, one cannot even imagine anything greater, and which, outside of God, no mind can succeed in comprehending fully.
Why then didn't Mary qualify to be our savior instead of Jesus?
Because Mary is not God. Man brought on hell by comitting an offense against someone of infinite dignity, namely God. But whereas he is able to offend an infinite God, he is not able to satisfy for that offense, because it is of infinite gravity.
This is because of the nature of the person in the respective cases.
In the case of sin, we are concerned with God's capacity to be offended (i.e. the capacity of the sin to be of infinite gravity).
In the case of propitiation for sin, we are concerned with man's ability to make up for an infinite offense.
As you can see, all God needs to do is be God; and sinning is easy for man. But as you can also see, man does not have the ability to do an infinitely good act. Only God, who is non-finitely Good itself, can always do only good. Thus, a Saviour must always be God as regards the problem of sin.
However, in becoming Incarnate, the Word made flesh (God, and so of infinite dignity), can propitiate God for the sins of men by undertaking a great Penance, that of the Cross. The suffering He endured pleases God so much that on its account, He is willing to allow others to benefit from its fruit, even though they don't deserve it. This is what the Mercy of God is. It is never ignoring sin or punishment even once, but rather gifting those who accepted such the satisfaction payment made by His Son.
Also, it is purely because Jesus stepped into creation and prevented the doomed hellish fate of all men that a Mary even exists, both at all (because God didn't have to allow Adam to live, but had forbearance for Christ's sake alone) and Mary as she actually came to be and is now known. This makes it impossible for a Mary to exist without a Saviour: hence she cannot even potentially qualify as a Saviour.
But as said above, no creature can propitiate for sins against an infinite God ultimately (I'm not talking about priests and such, who as intercessors appropriate God's mercy—they simply cannot effect mercy from God, they have to tap into it from elsewhere, namely doing what He says, which we learn in the New Testament, was a way to not go to Hell until He who made such forbearance possible actually came).
