My personal thoughts...

Yes, his ship was taking heavy damage from Mutas, but nothing that the shields couldn't cover up, or that another Carrier could've taken care of.

However, I believe that with his skill with both Templar AND DT energies, he could've stood outside his ship and channeled the DT energies into his body and then through the ship, so that it would've destroyed the Overmind AND left Tassadar still standing alive (albeit heavily damaged, this plan would require him to basically jump ship a few seconds before it hit the Overmind...)