can we do muththishi pchkhan before navkarshi time

If you mean taking a general pachkhan before Navkarsi, then yes, that can be done. Many vows can be taken in the morning before eating or drinking. But Navkarsi pachkhan itself is specifically observed at Navkarsi time, which is 48 minutes after sunrise; it is not completed before that time. ( jainknowledge.com)

So in simple words:

• Before Navkarsi time: you may take other morning pachkhan or vows.
• Navkarsi pachkhan: take it at/after Navkarsi time, not before. ( jainknowledge.com)

If you meant a specific pachkhan by “muththishi,” I can explain that exact one too.