My feeling is that it refers to ‘vital’ imagery.
Here he is mainly referring to Bankim’s imagery in which Sri Aurobindo finds ‘beauty, terseness, strength and sweetness’. The hideousness is not about Valmiki’s Ramayana. It is simply a literal portrayal of Ravan which is often done quite grotesquely as a purely vital image as we find in the film Adipurush. He is referring to contemporary literature during the time of Bankim not a judgment on the literary style and the Rasa flowing through Valimiki whom he ranks among the three greatest of all times in Indian literature, the other two being Vyasa and Kalidasa.
Affectionately,
Alok Da
