I always kind of pictured her as a plain (so that all she can do to towards makeing herself attractive is fuss about her clothes and hair, etc), and a "busy" woman - one of those people who always seems to be in the middle of something, but never really doing anything (although more a harmless prattler than a real busy body, if that makes sense).

Really, there is a lady in my acquaintance who reminds me a lot of Mrs. Allen so that is always who I picture - always "working" on something that no one else put half as much thought into (in Mrs. Allen's case, her appearance); never really noticing anyone else's discomfort / frustration / hints etc (like not doing much to help Catherine to dance, and later there are other examples).