Bilbo was aware that asking a child about portals probably wasn't fair to the child, but this particular one appeared to be willing enough to answer his questions. He was grateful for that fact. And the answers he received made a fair amount of sense, so he didn't think she was making it up. Madison Valley seemed strange to the point where her answers didn't seem out of the ordinary.
"Interesting..." he said, after he considered her answers. "And you received this Pokemon from a portal. ...so it isn't native to Madison Valley." He hadn't been positive about the Pokemon's being native to the area -- being new to the town did that to a person.
"...thank you for answering," he told the girl. "I know you didn't have to." He was grateful, truly. "My name is Bilbo, by the way."