is it just me who doesnt understand the ranking stuff?

Do you mean in general or here? Guideverse stories have ranking similar to those stories that have a gamified setting where S class is the best and F class is the worst (sometimes C class is the worst and they don't have F class).

In these settings, usually, espers and guides are of the same rank so an S class guide like ML is expected to be woth am S class esper. That is why people say he is wasted on B class esper MC because MC could've been guided by a B class guide and ML could've helped someone "better".