There is an appealing middle ground between "kind of a skank" like Mary Crawford and "sickly and irritating" like Fanny. Clergymen's wives do not have to be sanctimonious killjoys. Even in Regency England.
There's no explanation of Fanny's poor health. Perhaps her impecunious upbringing may be blamed, but she arrived at Mansfield at an early enough age not to be permanently afflicted with a vague malaise.
She also doesn't do anything. At all. On the plus side, she's not interfering in the way Emma is, but, on the minus side, she's unremittingly boring. Kind, and dutiful, and self-effacing, but dull as ditchwater.
Virtue she has, in plenty, but verve might be nice. You know, once. Maybe. Please.