I would think there's undoubtedly something of Rosie about Helen, but she can't be anywhere near as dim or she wouldn't have been able to hold down a job on Corrie for many years. There's an awful lot of lines to learn every day for actors who are involved in big storylines (which Rosie often used to be).
Also, it seems absurd now but at one time Rosie was supposed to be a reasonably intelligent girl for whom Sally had big plans (remember her pushy parent phase? :D), and from what I remember Helen wasn't totally unconvincing when playing her that way.