Certainly you can understand that this concept is highly subjective and ultimately what we are talking about is likely semantic. But I think that in order to pass a judgment like this, one must first be familiar with the entire body of work and, two, have the trained perspective to identify genius in these disciplines.
I am neither but I do know that summarizing Prince as "wrote a few good songs" is a dramatic understatement. Prince is often noted as one of the most complete and accomplished artists of his time:
- Musician: Prince wrote most of his songs on the piano which he began playing as a young child, but he was a brilliant guitarist - the kind of guitarists that other guitarists often say is one of their favorite or most impressive. On a stage of other musicians at the R&RHOF induction of George Harrison, Prince's delivered one of the most memorable live guitar solos people still talk about. Billy Gibbons described Prince's guitar playing as "sensational - otherworldly"
- Songwriter: Prince is said to have written between 500 and 1,000 songs. He recorded 39 albums and sold over 150 million. Prince had a handful of #1s, a dozen or more top 10s, and some he wrote of the most memorable songs of the 80s for others (Nothing Compares 2U, I Feel for You - Chaka Khan, Manic Monday, etc.)
- Producer: Prince produced his own music from very early on and throughout his entire career. Much of his breakthrough quality as a club music artist early on and then as a mainstream artist is owed to his skill and inventiveness as a producer. He even produced George Clinton records, that's saying something.
- Cultural force: Prince was unique throughout his career. He didn't really have a genre and within the genres that he was associated with he always had his own unique space. He had a unique fashion throughout his career that was often emulated. He had a stage presence, could dance and then deliver an epic guitar solo all in the same series of moments. Purple Rain was an entire cultural movement that has no pier except Hard Days Night. He won numerous Grammies but he also won two Oscars for music in a film. Purple Rain is in the significant film registry at the Library of Congress.
Do those things make him a genius? I don't think that's a controversial claim - though again, it's so subjective and depends on what makes a genius. I dunno.