When using earth or lime plasters, it is not necessary to use any mesh or lath. There are advantages to using mesh nonetheless, and there are disadvantages as well. The biggest advantage of using the mesh is the ability to shape the walls and reinforce the plaster in the process. The biggest disadvantage is that more plaster must often be used to even out the walls, especially if the mesh is rigid like a welded wire mesh, for example.
The rigid mesh will hold an even plane even if the surface of the bales undulates beneath it. This leaves deep areas that need to be filled with mud in order to remain even with the swells. In such cases, it can actually be more frustrating to use the mesh than it would be to plaster directly onto the bales. If you plan to place your mud directly on the bales, be sure you do a good job of weed whacking. Any loose straw will make the process more difficult as large sections of plaster will get too heavy and simply peel away from the wall. That is a mess and very annoying.
So either way, you can end up frustrated! Plastering is hard. Don’t let anyone tell you it isn’t. Once you get the skills from several projects, it gets easier, but it is nothing less than an art that must be learned and improved with time and practice.