> As I understand it, libel is construed as untruthful statements that
> damage a company in some way. But, what if someone's personal opinion
> is that a company is bad in some way, and the company loses money as a
> result? Can they be sued?

Libel is a statement that tends to damage someone's reputation that is made to someone other than the target of the statement. Truth was historically a defense for liability for liable (although in many circumstances in the United States is has been made part of the prima facie case that must be proved by an accuser).

Ordinarily a libel must be a statement of fact, and a mere opinion is not actionable. But, sometimes an opinion so clearly implies a closely related statement of fact in a particular context that the implied statement of fact arising from the statement of opinion, if false, is actionable.

Anybody can be sued, but they will not have a viable cause of action and their suit will be dismissed, if the alleged statement is truly a statement of opinion, even if it has consequences for a person's reputation and causes them economic harm.

For example, if I say that Calvin Klein clothes look better than American Outfitter's clothes, and people care about my opinion and purchase fewer American Outfitter's clothes, I have not committed actionable libel (even if I don't sincerely believe what I said since it is an opinion either way).