。
Yumi's hobby is singing pop songs.
Literal
Yumi-of hobby [topic-は] pop-song [object-を] sing-[nominal-こと] is-[polite-です].
趣味はXことです ('my hobby is X-ing') is the textbook hobby sentence — uses こと-nominalization to turn a verb into a noun phrase the copula can predicate. 流行歌 ('popular song, hit song') is a slightly old-fashioned word; modern usage might say ポップス or 流行曲. The ~こと nominalizer here is interchangeable with の but こと feels more formal in this pattern.