i think to be counted as a genius you have to really innovate in music < that can be hard to pin down as everybody is building on what has gone before. from a pure theory point of view i would expect academics have had a go at pinpointing people who have really broken new ground on the timeline. Easier to do with classical music and jazz i expect.
For me the best female singer (okay my favourite female singer) is Chaka Khan , but much as she understands music theory deeply it would be hard to call her a genius - its the quality and technique of her voice that makes her stand out.
And then you've got people like Prince who can play many instruments, sing, obsessively write songs all day every day, create piles of classic popular songs...hard not to call him a genius really, though did he innovate? To some extent I think yeah, he always had his own sound for sure - though I wouldn't put him in the list of key innovator necessarily, more he admired certain people before him and combined elements from them all - with his own personality too for sure.
stevie wonder is simliar to prince on all of the above points, but I think he was more innovative than Prince, in his 70s period especially - there are "stevie wonder chords" , melodies and chord progressions that could only be written by him.
And yeah I think you have to write material to be in the genius camp...Elvis Presley was a great performer, but singing other peoples tunes - I dont think thats genius, I think thats talent and charisma.