This is a case where the disorder may mean different things in different contexts, but I don't think there can be any denying the fact that frequent memory loss retards normal brain function. Nevertheless, calling it a symptom of the disorder suggests a causal link between the two, and I don't think it can be demonstrated that suffering from retardation leads to memory loss. Perhaps in some cases it does, but not enough to qualify as a symptom.
Although frequent memory loss may indicate that a person has brain damage or an injury related to their neurological systems, it does not constitute a symptom of mental retardation. Such conclusions would need further evidence and diagnosis to be acceptable. The question is not clinically sound, nor is it scientifically proven.
I do not think that frequent memory loss should be accepted as a form of mental retardation symptom. I think that it is something that is considered as a form of mental illness. There is nothing about it that is similar to it being a form of mental retardation and mental disorder.
Actually, many smart people have bad memories. I think most people would say smart people are far from having the condition of mental retardation. There is only so much room in people's brains. They have so much knowledge in their head that they can never remember little things, like where they put their car keys.
There are many reasons why people would have a bad memory or would have memory loss, and there are far better ways to tell mental retardation in people without this. The concept of the absent minded genius is a classic trope, showing that brilliant people frequently have memory issues also.