Comparative adjectives are used to compare a noun to another noun such as this truck uses more gas than this car.
Superlative adjectives [in this case, “most”] are used to compare three or more nouns and show which is best such as hot, hotter and hottest.
Since there is only One God, we would not use the superlative to distinguish Him among many gods, as being the “most” holy God.
However, we may rightly use the superlative form to distinguish the Pope as “most holy Father”, as being “most” among others whom we call “father”, such as our priests.
Sirach gave a great response on the semantics of comparison, but I think this becomes even more clear when we consider the original language of the scripture (Greek/Aramaic) and the reason why it is different to call a priest or parent father than it is to call God Father (note, as well, that the Bible tends to make this distinction by capitalizing a letter). The Bible actually uses two different words to make this distinction.
When Jesus says “call no man Father,” the word that He uses for Father is “Abba.” This is a combination of Greek and Aramaic, the Greek “-ba” which is the familiar form of the word father, like calling someone dad. The Aramaic honorific “A-” at the beginning has the effect of elevating the word to an honorific… more of a singular name than a title. So Abba is no longer just a title, it’s a singular… there can only be one Abba (hence why Christians here were admonished against calling Caesar Abba, despite the fact that Caesar wanted to be represented as part god on earth).
By contrast, when Paul says that we have few fathers on earth, but that he has become the father of the Corinthians (1 Cor 4:15), the term used is “paternas.” This is simply calling someone by a title of “father” (lower case F).
So God (Abba) is THE Father, the one Father who is a Father by nature of His very being, and from whom all fatherhood is established. Priests, parents, and even the pope derive the rightful role of their fatherhood from God’s Fatherhood. In other words, Sirach’s post is especially apt because God’s Fatherhood and man’s aren’t even in the same category.
As to why the pope is the most holy father? If Holy refers to being “set apart,” then it makes sense that the Pope is the MOST Holy, since the Pope is the only person who is singularly imbued with magisterial teaching authority (meaning that the pope is the only person who can teach universal doctrine with us being assured that said teaching is absolutely true). This special Charisma of the office is singularly distinctive, hence “MOST Holy.” It has nothing to do with the righteousness of the man in the office (so it might be worth pointing out that Catholics, unlike many protestants, consider there to be a difference between righteousness and holiness… they’re not exactly the same thing, although they often go together)