'Infinite dignity' is a phrase that, shall we say, can be used unhelpfully and in a slipshod manner. On the other hand, St. Thomas does make very beautiful, clear, illuminating, and edifying use of this term, in describing a 'sort of' (quandam) infinite dignity which the three greatest possible things that can 'come to be' (fieri) have, namely the Sacred Humanity of Christ, created beatitude, and the Blessed Virgin Mary, precisely because of their relation to the infinite good, God.
humanitas Christi ex hoc quod est unita Deo, et beatitudo creata ex hoc quod est fruitio Dei, et beata virgo ex hoc quod est mater Dei, habent quandam dignitatem infinitam, ex bono infinito quod est Deus. Et ex hac parte non potest aliquid fieri melius eis, sicut non potest aliquid melius esse Deo.
My translation:
'The Humanity of Christ, from the fact that it is united to God, as well as created beatitude, from the fact that it is the enjoyment of God, and also the Blessed Virgin, from the fact that she is the Mother of God, have a certain kind of infinite dignity, on account of the infinite good which is God. And for this reason, there cannot come to be anything better than these, just as there cannot be anything better than God.'
(St. Thomas Aquinas, Summa Theologiæ, Prima Pars Q.25 a.6 ad.4)