I'm sure I've all but lost friends by maintaining that, despite their love for it, I always saw Stanley Kramer's 'It's a Mad, Mad, Mad, Mad World' as more of an exercise in anti-comedy than humor.

This quote is saying that the speaker has lost friends because they do not agree that the movie “It’s a Mad, Mad, Mad, Mad World” is humorous. The speaker thinks that the movie is an exercise in anti-comedy, which means that it is more about making fun of comedy instead of actually trying to be funny. The speaker’s friends may love the movie but still disagree with the speaker; this is why the speaker has lost friends by speaking their opinion.