I don;t think these are sfx, they are the actual shouts of the cat. My Japanese is awful, but if there is only one word I’ve learnt from reading 8 and a half volumes of Yotsuba, it’s ぎゃ! It’s often used as a shout, especially when someone is in shock in some way.
I think the first one here, with the “m” at the start, shows the movement in the voice from “meow” to shout! Very funny. (Even though I am determined to dislike this rotten cat and its awful speech patterns!)
