I used drywall mud to make quick, easy work of sanding but I wasn't making making a plug to pull a mold from. I was making the backside of a sacrificial form so I knew the top surface of my plug would be the inside of the actual flare and hidden from view.
If these things are going to be plugs, I think it's probably best to stay on the Bondo track. Drywall mud is quick to work with compared to bondo but it's also much more fragile and shrinks/cracks if applied too thick.
If I was you and I knew my plugs were well supported while laying up the mold, I'd use drywall mud and then paint it to get a nice smooth surface for molding. If your plugs will be off the car or in any way floppy when you lay up the molds, I'd use bondo for its strength.