If it is a mere opinion, it would not constitute libel. For something to be libelous, it must be an assertion of fact - it must also be false and tend to injure the reputation of the party within the community.
Isn't every opinion an assertion of fact? If I say "American cars suck" or "Apple mistreats its developer community", I'm expressing what I believe to be the truth, not just my opinion.
What's the distinction between opinions and assertions of fact?
You make an excellent point, though, in picking up on this issue. No less an authority than the U.S. Supreme Court (in the 1990 Milkovich case) expressly rejected what it called "the creation of an artificial dichotomy" between fact and opinion and held that statements couched as opinions can be libelous if they imply false and defamatory facts.
So, if you say, "In my opinion, politician x is a drug dealer," your statement could be found to be defamatory.
For a good discussion on this, see http://www.citmedialaw.org/legal-guide/opinion-and-fair-comm....