What is Hemingway's attitude toward the main character, Harold Krebs, in "Soldier's Home"?
We can only know Hemingway's attitude toward the protagonist in "Soldier's Home" through the narrator of the story. You should be careful of associating a narrator or speaker with the author. Authors create personas to tell their stories for them.
That said, the speaker in the story is sympathetic to Harold Krebs. He certainly understands Harold better than any other characters in the story. The other characters all have their own ideas of what Harold should be. The parents want him to be a typical, middle-class worker. The townspeople want him to shock them with war stories even better than the ones they've heard before. Even his sister has categorized him: she wants him to be her "beau." Everyone has ideas about what Harold should be.
But Harold has no ideas. Harold has suffered through a war. He seems to have enjoyed living in Europe after the war, but he is a cipher in his home town.
One can probably assume Hemingway was sympathetic to his created protagonist, Harold Krebs. But unless you have information outside of the text (such as similar stories in which similar sympathies are shown), you would probably want to talk about the speaker's sympathy, rather than Hemingway's.