They're not contingent upon each other. You can be charismatic and not not be able to do it on camera. Ryan Renolds? Channing Tatum, Colin Ferrell are charismatic personalities with horrible screen presence.
Clark isn't a charismatic character, he's dull, he has no charm, clumsy, nothing special or intimidating, nothing that would appeal to anyone, as a character he's overlooked among everyone. Yet Reeve still made you pay attention, even against the full-of-life Margot Kidder. That's why I said "per se".
Which is why I prefer the Dean Cain version of Clark Kent. His Superman was not great but the show wasn't about Superman, so it was alright.