There are two questions here.

• d’sa is a simple elision of de sa.
• ne is omitted because it is fading out in oral French. And we already have a question about that

So this is a common realisation of « ce n’est pas de sa faute » in oral French.

I’m assuming that sentence was part of a dialogue. The character is speaking in a familiar register of French, so the author is not writing in proper French to get that fact across.

In oral familiar French, it is common to drop the final e and kind of glue the next word to it. So d’sa is actually de sa elided together.

It is also very common to drop the double particle negative in oral French and omit ne altogether. You are correct, it should be Ce n’est pas, but with the ne omitted, it does become C’est pas.

It’s not his/her fault! But it’s a very familiar expression, the correct one would be ”Ce n’est pas de sa faute”.