That really depends what they lie about
In the United States, there's no general law against lying. The fact that a statement is false doesn't inherently strip it of protection under the first amendment. Public figures lie to the public all the time. That's why news companies have fact checkers.
Was it defamatory?
It is, however, illegal to defame someone. If someone makes a false statement of fact (that is, not an opinion) about a person or company, they may be liable for that. Whether they are liable for that depends on a number of factors, including whether the target is a public figure (see New York Times Co. v. Sullivan), the speaker's knowledge of its falsity, and whether the target was damaged by it.
Was it part of some other criminal scheme?
False statements to the public could be part of some sort of fraud, for instance. Pump and dump schemes, for instance, are illegal.
Was it under oath?
Lying under oath (such as when testifying in court) would constitute perjury, which is a crime.
There are many other situations in which lying could be a crime (such as lying on your taxes), but these are the main ones I can think of that would be about lying to the public.
In this case (I'm unfamiliar with the details of what he said, so I'm just going off your description), I can't immediately think of any reason that could lead to liability. Mocking and calling something a "nothing burger" is pretty clearly an opinion, not a false statement of fact. I'm not aware of any securities law against saying you don't like something you're actually invested in (though I'm not especially familiar with securities law).