I heard somewhere that Ashwathama was a partial incarnation of Mahadev Shiv Ji, is that true?

The bold and lotus-eye Ashvatthama, of extraordinary courage, abuser of adversaries and dreadful to foe armed forces, was conceived from three pieces of Mahadeva that converged into one—yama, kama and krodha.

BORI CE interpretation by Bibek Debroy Sambhava Parva.

So as should be obvious he was conceived somewhat from Kama, krodha, and Yama. So no he was not Partial Incarcation of Mahadev Shiv Shankar .

He was mostly conceived from kama , krodha , Yama and rudra.