Can you name some artists with average or mediocre vocal talents that sustained long musical careers?

Of course not, but vocal talent does equal better musicians wanting to work with you and that = good music.

Britney.

Madonna.

Madonna, Britney, Rihanna.

Not at all. Lea Michele's debut is unfortunately kind of weak, which has nothing to do with her outstanding vocal ability. Good singers can make bad music just as easily as bad singers can make good music, it's just about knowing how to use the talent you have.

Mariah Carey, Rihanna, Britney. They each aren't very strong vocally but have had careers

Well Madonna is a fantastic musician.
Good Musician=/= good vocalist

In before someone says Katy She's flawless!

Katy doesn't have a long career.

Less on Mariah. Not when #Beautiful was better than anything released in 2013 (pre Dec 2013).

But why are you equating making good music with having a long career?

But she will

Britney , Madonna , MJ , .........

DEMI (the album) proves that vocal talent =/= good music tbh.

I think it's a really thin line.

I mean take Lea Michele as an example. GREAT voice, but the material they gave her for the debut record is just good at best. The record turned out good just because her vocal skills are amazing and she can put emotion into it.

But yeah, vocal talent doesn't guarantee great music.

Fifth Harmony

No, and I hope people stop assuming so. I also hope people stop saying personal/relatable makes a song automatically good.

Never. The talent has to be used and applied properly.