Isn’t the curse to Niranjan to devour one Lakh Jivas daily a sort of punishment to the soul?

No, this is the punishment to the Mind, instead.

Jeev Ke Sung Munn Kal Rahayi, Agyani Ner Janat Nahi.  

Kaal in the guise of Mind is residing within
Individual beings, but the ignorant persons don’t realise it.