Mycoplasma organisms are pleomorphic in nature due to

Mycoplasma organisms are pleomorphic due to absence of rigid cell walls.
Mycoplasma is a genus of bacteria that lack a cell wall around their cell membranes. Peptidoglycan (murein) is absent. This characteristic makes them naturally resistant to antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic.
Due to the absence of cell walls these organisms can change their shape and are pleomorphic as they cannot be identified as rods, cocci or spirochetes.