Robert De Niro is a native speaker but does not speak very smoothly. In interviews he is painfully shy, his sentences are short and he is often lost for words.
Al Pacino, Johnny Depp, Christopher Walken etc. are not that good either. They all are native speakers and know English grammar and a lot of words but their language lacks fluidity. In movies, actors memorize their lines and do a lot of takes before they get it right.
People like Steve Jobs and other public speakers practice a lot before they get on stage and give their presentations, not to mention the teleprompters.
Bill Clinton's eloquence is unparalleled. George W. Bush, on the other hand, makes use of fillers like um, uh, like, y’know, well, etc. and
Barack Obama is almost lost without his teleprompter
But that doesn’t mean they are less talented, less intelligent or less educated. It means that being fluent, being able to speak easily, well, quickly and without many pauses, is a very different skill. There are a lot of smart people who are not the best communicators.
At the same time, there are people who are very good at using language, very smooth and expressive, but who are not all that... well, you be the judge