Yeah, there's definitely a difference between being good on the mic and being charismatic. As has been pointed out there have been a number of wrestlers over the years that were charismatic as hell, but weren't that great on the mic. A couple of examples...
Bret Hart: With the exception of his 1997, Bret was never very creative or smooth or clever on the mic. His promos were pretty basic. Definitely not bad, but basic. But it didn't matter. Bret had the "it factor" where the fans just instantly got behind him, even back in his Hart Foundation days. Bret is someone that you could relate to. His character what that of an honorable, noble man that believed in competition and winning, and being a hero. He didn't need promos to be over with the crowd. From his mannerisms like his walk to the ring, the way he carried himself as a no-nonsense type of wrestler, people got behind him. Not to mention he was smaller, more athletic and put 100% into every match he had, and fans appreciated it. Many female fans considered him a sex symbol, and guys liked him because he was tough, no-nonsense. Bret's probably the biggest example of a guy who wasn't that great on the mic, but oozed charisma.
Jeff Hardy: Jeff Hardy is another one. He's another person that people relate to, because he's small and gives 110% to every match. He did the craziest things and the fans loved it. And again, he's not great at all on the mic. He's just relateable to people.
Brock Lesnar: Again another guy that's not the best on the mic, except for taped promos for matches, where he's phenomenal. But live on the mic, he's always had Paul Heyman or someone else to do the talking and he doesn't need it. Wrestling fans love a beast-tough guy type of wrestler, and no one demonstrates that more than Lesnar. Not good on the mic, but people love him because he's so good in the ring, and because he completely dominates.
Goldberg: He's exactly like Lesnar. He didn't say much at all, he just went into the ring and kicked ass. And wrestling fans love wrestlers like that.
Early Undertaker: Taker is actually a great promo guy, particularly in the late 90's and early 00's as the American Badass character. But early on his career, he didn't say much though what he did was awesome and perfect for his gimmick. But again he didn't say that much, it was mostly Paul Bearer talking. But his character was so mesmerizing and unique that fans got behind him. Fans didn't care about his promos that much. They cared about how invincible he was in the ring and how he dominated.
Bruno Sammartino. If you go back and watch is promos he wasn't exactly lighting the world on fire with his mic skills. He was your typical humble, heroic figure when it came to his mic skills. He certainly wasn't a Buddy Rogers, Killer Kowalski, or Billy Graham on the mic. But again he didn't need it. He was able to connect to the fans and he became their hero.
So the proof is in the pudding that mic skills are not the end-all be all on a wrestler getting over. It definitely helps and being great on the mic does make a wrestler a better overall performer, but yeah it's not everything.
As for which is more important, great mic skills or having charisma to get over with the fans? Obviously it's the charisma, because as has been pointed out, there have been plenty of wrestlers over time that got over and became big draws without being creative, unique and awesome on the mic. But every wrestler that has ever been over has had to have a type of charisma to do it. Either by their-in ring work, their character, their mannerisms and presentation of their character, etc.