Yes, it’s perfectly fine to capitalize pronouns for Mary as well as God. For that matter, it’s perfectly fine to capitalize pronouns for other people too In German, it is the custom of the language to capitalize “You” when writing a letter, even if its a letter to a lowly peasant In other languages, there doesn’t even exist capital letters or lowercase letters. Also in English, personal pronouns are still capitalized even when speaking to a mere mortal like “Your Honor” or “Your Holiness” or “Your Eminence” or “His Honor” “His Holiness” “His Eminence” “His Excellency” et – this includes both ecclesiastical and secular titles.
In the past personal pronouns for the Pope were also apparently sometimes capitalized – and not just with the style “His Holiness” but personal pronouns in general, at least in some formal letters. Actually, this is still true today. If you look at one of the early homilies written by present Pope Benedict XVI – as written in the original Latin, not in a translation which leaves this out – he refers to his person we the royal, capitailized We, Our, etc.
So if it’s kosher to do it for a Pope, it’s surely kosher to do it for the Queen of the Pope.
It is natural however to feel strange when first seeing it since in your mind you may have associated with signifying supreme divinity. But with time, as long as you are not overly-scrupulous or somehow hostile to Mary’s grandeur, you will get used to it.
Joseph is correct about the CCC. However the CCC does on about two occasions capitalize a pronoun for God. I don’t know about Mary. The reason why it does capitalize on a couple rare occasions is to remove grammatical ambiguity (i.e. some may wonder if the pronoun’s antecedent is God or some other term in the sentence like man – and so capitalizing it removes any potential ambiguity)