there is a slight, but important difference, between these two forms of bacteria.
Mycoplasma are cell wall deficient bacteria that have ALWAYS been cell wall deficient.
L-form bacteria are more insidious than Mycoplasma because they are able to shape shift between a normal bacterial state, with a cell wall, and a cell wall deficient form. Organisms such as Borrelia Bergdorferi, are l-form because they actually exist in multiple different forms, including a cell wall deficient form. same with chlamydia pneumonia.
Mycoplasma bacteria never have a cell wall, therefore they are not technically classified as l-form.