This might seem like an odd combo, but hang with us for a second. First off, the narrator and some of the major characters, namely Mary, are very witty. Mary often makes puns, or plays on words, and ridicules other characters, while the narrator often makes fun of characters and points out the absurdity in certain situation. This wit often helps us laugh during scenes that would otherwise be pretty serious. For instance, in the scene where Sir Thomas nearly grounds Fanny for life after refusing to marry Henry, we get a rather hilarious description of how Sir Thomas finds Fanny's sobbing fit kind of awkward (32.34-43).
However, this wit is usually not cruel. Even the characters who are ridiculed most often, like Mrs. Norris, get some sensitive treatment and a representation of serious feelings. Overall, the narrator takes a pretty sympathetic approach to nearly all the characters, giving us insight into their feelings and motives, and often their sufferings.
This brings us to detached. Despite the often affectionate wit and outright sympathy in tone, it's often hard to tell who the narrator is favoring, praising, or critiquing. We are presented with a cast of very flawed characters and often we don't have a clear hero, or we at least lack a hero who we consistently approve of. Fanny is especially problematic when it comes to figuring out the book's overall tone. Fanny gets a lot of sympathy, with frequent comments like "poor Fanny" from the narrator. But the narrator also subtly critiques and ridicules Fanny at times, pointing out things like Fanny's (over)confidence in her own "purity." So we can call the tone detached because the narrator often tells us the story without strong emotions, or at least without easily definable or clear emotions. The narrator is hard to pin down here.