Did different opinions matter during WWI, and were there critics of the war criminals (consider the United States only)?

In the eyes of the law, yes, critics of WWI were criminals in the United States. Eugene Debs is a prime example. He spoke out against the draft and business's close relationship with government—he was given a prison sentence for speaking his mind.
Your definition of "matters" is important in considering this question, but it is important to note that there was some opposition to WWI. Some Progressive groups did not want to get into the war, as it would detract from their domestic agenda. Some immigrant groups did not want to act against Germany or Austria-Hungary. Others, such as the Irish and Polish, did not want to help Britain or Russia. Pacifists opposed the war on moral grounds. In the end, government clamped down on directly opposing the war: it was illegal to impede munition work or the draft.
Citizen groups were often more harsh than the government in their support of the war. Some groups took it upon themselves to burn German books in libraries. People who did not work fully in munitions or sign up for the draft were called "slackers" and treated as outcasts by their neighbors. While government restricted free speech during this war to "preserve democracy," it was really citizen groups outside of government that proved to be the most restrictive in suppressing dissent.