The implication is that Mr. Grimby is evil.
Your confusion probably results from not understanding the phrase "suited to." In this case, "suited to the night" means "benefits from the night."
Compare this to people (like house painters, dressmakers and jewelers) who benefit from working in daylight or in lighted places.
Who benefits from the night? Someone who who wants to do evil and doesn't want to get caught benefits from the night.
Why is "sleepwalker" the wrong answer? A sleepwalker doesn't benefit from sleepwalking, so he doesn't really benefit from the night either. The night just coincidentally happens to be when he sleeps, but the night is not a benefit to the sleepwalker, unless his GOAL is to sleepwalk.
Sleepwalking is an unconscious activity of which the sleepwalker is unaware, so the sleepwalker can't be said to be benefiting from a circumstance (night) that he doesn't even know is occurring and that doesn't help him to accomplish anything or achieve a goal.
Evil people benefit from and are "suited to" the night, because evil people and their evil deeds are less likely to be observed and discovered in the darkness of the night. They consciously wait for the night, just like animals hunt at night, because that's when they are least likely to be detected and when their secret, unannounced intentions are least likely to be frustrated.