Constitutional law, and particularly the First Amendment, is a complex body of law that many law students never master even after one very intensive semester of it. I already have a poor reputation of posting indiscriminately about all things having nothing to do with aviation, so for fear of being banned outright, I guess I won't go into a long disquisition about this topic.

However, on the off chance that you are geniunely interested, I suggest you look up and read the case of Gertz v. Robert Welch, Inc., a 1974 case where the US Supreme Court discusses whether opinions can be defamatory. The newer case of Milkovich v. Lorain Journal Co. held that a statement must be provably false in a cause of action involving libel.

Note that Mesa's lawsuit is not limited to defamation, but apparently alleges trademark violations. Having not seen the pleading, I cannot tell what other allegations it makes and also do not know how the respondent answered.