Yes, I understand what you are asking. "Through no fault of your own" means not due to misconduct, but it's helpful to look at the question in terms of, "could the employee have prevented the problem?" Certainly, drinking on the job and the other examples you cite will be misconduct, and the discharged employee had better not count on unemployment compensation. But other types of less serious conduct will also me misconduct, such as insubordination to the worker's supervisor, arriving late to work, or chatting with coworkers while ignoring waiting customers.
So, to look at your question about employees who are not top performers, could the employee have prevented the problem? The employee who does lousy work because they are too busy using the company computer to socialize on Facebook, or the employee who has been told to complete a task in a certain way but takes an unauthorized shortcut that results in inferior work is arguably dischargeable for misconduct. The employee who is not meeting his or her sales quota because he or she is just not talented at that (I'm no good at sales, myself. It's truly a gift) or the employee who is trying to do the job but is just plain in over his or her head, certainly is a target for a layoff, but will probably qualify for unemployment.
Put very simply, if you "can't", you are likely to qualify for unemployment. If you "can but just won't", probably not. Hope that helps.