Q: Thank you for all the great work you do for the Church. Are we permitted to use a Common Weekday preface on a Sunday in Ordinary Time if it seems more fitting? Thank you.
A: I don’t see why not. You refer to them as “Common Weekday prefaces” but the missal just calls them “Common Prefaces.”
They are catchall prefaces when “a preface related to a specific liturgical time is not indicated,” as the rubric on top of each reads. The restriction pertains more to Advent, Christmas, Lent and Easter than to Ordinary Time.
The only reference to them in the GIRM is in 365b, where they are given as examples of the prefaces useful with Eucharistic Prayer II, which it also recommends for weekdays.
I don’t see anything clear one way or the other, but it seems to me you could choose one for a Sunday.