It's more that mewn is used for indefinite nouns, i.e. where you would say "in a ..." (e.g. mewn siop "in a shop").
But proper nouns are definite pretty much by definition (there's only one Shrewsbury).
You would also use yn with a common noun that is definite, e.g. "in the school (= the one that we had been talking about earlier)" as opposed to "in a school (= that had not been mentioned before)".