Miss Steele is a harmless creature who somehow manages to do a lot of harm, if that makes sense. Sure, she's silly and annoying, but, like many of the characters here, she's not bad at heart. What she suffers from is a severe lack of both common sense and discretion, especially when it most matters – after all, it's Miss Steele who reveals Lucy and Edward's engagement to Mrs. Ferrars. While anyone else could have figured out that the revelation of the secret was none of her business, and a bad idea overall, Miss Steele genuinely thought she was doing the right thing.
Really, Miss Steele is just well-intentioned ninny – she's at her best when she's allowed to gab on and on about "beaux" and gossip, and at her worst when she's entrusted with important information. She's not wicked or anything (unlike her manipulative sister), she's simply not the brightest star in the sky.