For those of us who are old enough to remember 1973, Ringo had 2 number 1 singles, and a number 5 single, with his album peaking at Billboard number 2 and staying in the top 10 for a long time. That's a whole lot of people thinking he didn't suck. And it wasn't because he had a great voice, it was because he used what he had really well on really great songs. And no, it wasn't just because he was an ex Beatle, though that certainly didn't hurt. It was just a good record.
There are plenty of cases where mediocre singers have big hits and big careers, just as there are tons of gifted singers who evoke no response from the record buying public. The trick is making what you've got work to emote great songs.
-RD