I didn’t post any fillet pictures, but the ice fishing fillet pictures someone else posted looked good.
I’ve filleted thousands of fish with guys who have filleted thousands more than me and I’ve never heard once heard of a mud line or back fat on walleyes or panfish. White bass have a mud line along the back of the fillet that we remove though. I had northern Wisconsin walleyes for New Year’s Eve dinner and my kids had walleyes for lunch today.
What state are you from? Maybe fish from warmer areas need different preparation, but I’ve never seen that in Minnesota, Wisconsin or Canada.