I think she's playing the part as a characature (sp) or panomime rather than a straight role. she knows what she's doing, have you seen those wigs she's wearing? It's almost Skakespeare.
Sue Johnston has shown that she is a good actress doing comedy eg Royale Family, and straight roles (Brookside, Waking the Dead, etc) but the scripts in Corrie are awful for her just now so she's doing the best she can!