People way over-emphasize this imo. There is definitely a problem of representation for darker-skinned woman. But the majority of female stars in South Indian movies are South Indian.

If you look at some recent Tamil best-sellers:

Bahubaali: Anushka Shetty(Tulu), Tamanna Bhatia(Maharashtra), Ramya Krishna(Tamil)

Ponniyin Selvan: Aishwarya Rai(Tulu), Trisha(Malayali/Tamil), Aishwarya Lekshmi(Malayali), Sobhita Dhulipala(Telugu)

Vikram: Gayathri Shankar(Kannadiga), Swathishta Krishnan(Tamil), Myna Nandhini(Tamil)

Varisu: Rashmika Mandanna(Kodava/Kannadiga), Jayasudha(Telugu), Sangeetha(Tamil)

Kabali: Radhika Apte(Marathi from Tamil Nadu), Sai Dhanshika(Tamil)

The vast majority are South Indian. South Indian heroines rarely stick to one language because of how limited their parts are(which is unfortunate), Tamil/Telugu/Kannadiga/Malayali actresses act in all Southern movies.

Is there a problem with dark-skinned representation? Yes. But frankly I think North Indians like to think that South Indians need to get North Indian heroines because they're better looking. Are there some North Indians in South Indian films? Of course. There are some South Indian actresses in Bollywood films as well. But this narrative isn't really true