Do Catholics believe that Mary is above all other humans in holiness?

First of all, according to Jesus, Mary's holiness is not due to the fact she's the Mother of the Saviour nor to her immaculate conception :

And it happened, as He spoke these things, that a certain woman from the crowd raised her voice and said to Him, "Blessed [is] the womb that bore You, and [the] breasts which nursed You!" But He said, "More than that, blessed [are] those who hear the word of God and keep it!" (Luke 11, 27-28)

Mary meets the requirements to be said blessed because She is the one who beleived to the Word of God and kept it in her heart :

Elizabeth was filled with the Holy Spirit. Then she spoke out with a loud voice and said : [...] "Blessed [is] she who believed, for there will be a fulfillment of those things which were told her from the Lord." (Luke 1, 41-45)

But Mary kept all these things and pondered [them] in her heart. (Luke 2, 19); His mother kept all these things in her heart. (Luke 2, 51)

The evidence Mary is crowned (Queen) in heaven and above all other humans in holiness can be found in Apocalypse 12.

Now a great sign appeared in heaven: a woman clothed with the sun, with the moon under her feet, and on her head a garland of twelve stars. (Ap 12, 1)

The verse 5 shows that this Woman is Mary. She is above all humans as Queen (crowned with stars) and as mother of all the christians (verse 17) :

She bore a male Child who was to rule all nations with a rod of iron. And her Child was caught up to God and His throne. (Ap 12,5)

And the dragon was enraged with the woman, and he went to make war with the rest of her offspring, who keep the commandments of God and have the testimony of Jesus Christ. (Ap 12, 17)

Some quotes from the Magisterium : SIGNUM MAGNUM starts with this affirmation :

The great sign which the Apostle John saw in heaven, "a woman clothed with the sun,"(Ap 12, 1) is interpreted by the sacred Liturgy, not without foundation, as referring to the most blessed Mary, the mother of all men by the grace of Christ the Redeemer.

And MARIALIS CULTUS says :

35. First, the Virgin Mary has always been proposed to the faithful by the Church as an example to be imitated, not precisely in the type of life she led, and much less for the socio-cultural background in which she lived and which today scarcely exists anywhere. She is held up as an example to the faithful rather for the way in which, in her own particular life, she fully and responsibly accepted the will of God (cf. Lk. 1:38), because she heard the word of God and acted on it, and because charity and a spirit of service were the driving force of her actions. She is worthy of imitation because she was the first and the most perfect of Christ's disciples. All of this has a permanent and universal exemplary value.

I also want to mention Redemptoris mater :

19. In the expression "Blessed is she who believed," we can therefore rightly find a kind of "key" which unlocks for us the innermost reality of Mary, whom the angel hailed as "full of grace." If as "full of grace" she has been eternally present in the mystery of Christ, through faith she became a sharer in that mystery in every extension of her earthly journey. She "advanced in her pilgrimage of faith" and at the same time, in a discreet yet direct and effective way, she made present to humanity the mystery of Christ. And she still continues to do so.

Mary's dignity and all of her privileges follow from her vocation, which is from all eternity God chose her to be His Mother, therefore this places her in a realm distinct not only from all the saints, but from all the angels as well.

The Virgin Mary, who at the message of the angel received the Word of God in her heart and in her body and gave Life to the world, is acknowledged and honored as being truly the Mother of God and Mother of the Redeemer. Redeemed by reason of the merits of her Son and united to Him by a close and indissoluble tie, she is endowed with the high office and dignity of being the Mother of the Son of God, by which account she is also the beloved daughter of the Father and the temple of the Holy Spirit. Because of this gift of sublime grace she far surpasses all creatures, both in heaven and on earth. At the same time, however, because she belongs to the offspring of Adam she is one with all those who are to be saved. She is "the mother of the members of Christ . . . having cooperated by charity that faithful might be born in the Church, who are members of that Head." Wherefore she is hailed as a pre-eminent and singular member of the Church, and as its type and excellent exemplar in faith and charity. The Catholic Church, taught by the Holy Spirit, honors her with filial affection and piety as a most beloved mother. - Dogmatic Constitution on the Church, Lumen gentium, 53

And she is holier than they because she placed no obsatcle to this unique grace from God, saying yes to God all they days of her life, faithfully corresponding with all graces:

And Mary said, “Behold, I am the handmaid of the Lord; let it be to me according to your word.” - Lk 1:38 (RSVCE).

Many saints and doctors of the Church consider that the initial grace in Mary was greater than the final grace of all other beings.