What if your employees are simply mediocre performers?
What if there was something that could motivate them to get better?
What if there was something that could motivate them to get better?
So ultimately if you could motivate them to be better, then by definition they would be mediocre performers.
Based on my personal experience, I would not be surprised if you were to show me a study that found a strong correlation between being internally motivated and being considered a star performer.
Besides, by definition, most of your employees will be average. Otherwise the average would shift. Focus on bringing the whole bell curve up by improving your systems and your management, rather than on terms you use for individuals. You're better off optimizing your company and your management to handle this diversity, since it's a natural result of a sample.
For example, the Ivy League colleges suffer rampant grade inflation due to these high-achieving students not being able to cope with getting less-than-perfect grades. This poor coping is a form of 'low achievement' that drives other 'low achievement' behaviors: reduced course load, dropping out, switching to less rigorous majors, etc.