Under plurality voting, third parties are objectively bad for the winning chances of whatever party they pull more voters from (and/or bad for whatever major party you personally would vote for, if you personally vote third party). That might be what some people meant by them being “bad”, even though they prefer one of those major parties to win (and cheer on anything that makes the other party less likely to win). Although it’s certainly possible that some people just think they are bad in general. People do have different opinions, after all.
Their worst sin in my opinion is actually how they split off the end of the Smaug storyline to be resolved in, like, the first five minutes of the third movie (literally before the title) rather than resolving it in the second movie. Huge pacing mistake.