Question

What does Roosevelt mean when he says a muck-raker is like a person who "fixes his eyes only on that which is vile and debasing"?

Answer 1

Roosevelt means that a muck-raker - a journalist- is that person interested in dirt or filthy things - muck-. That person will do anything to get filthy information and to get it,  he / she will use a  rake to scratch the soil - or reality -  to pick up or invent the dark side of  humans. Roosevelt is furious because a group of journalists have disclosed controversial issues from his government. He shows his anger through   this speech on "The Man with the Muck- Rake" (1906).

Answer 2

he is comparing muck-rakers with people who focus on negative things.