Personally, I think Reshmi is doing a decent job as Mallika. She looks and acts psychotic because Mallika is an obsessed maniac!!!!!! She shamelessly lusts after a married man, and is quite proud of her 'subtlety'! lolz Reshmi was very good in her scene with Ayesha
Plus, it's wrong to compare Sanjeeda's and Reshmi's performance, as I have noticed often, because Ayesha and Mallika are actually quite different.
Ayesha was a sweet girl who turned evil due to circumstances! She did harbor true feelings for Milind, at least for a while, and definitely cared for him till her last breath!
Mallika is simply insane from the very beginning, and as long as she satisfies her temporary lust, she doesn't give a damn about whether Milind rots in jail or hell!
Ayesha had a gentle heart that turned into stone. Mallika, on the contrary, is a heartless ANIMAL! I think Reshmi is alright for this role, and might get even better, since, apparently, that psycho is here to stay for at least couple months!!!!!