A definition of "clinical" is "of or based on direct observation of patients". If "Mean clinical score" is related for example with the rult of a test of a medicine, I think that this literal transaltion is correct.
I do agree with Gabriel, The word mean I interpret it here as media (aritmética), promedio. However, the word score I'd translate it as resultado. The word Puntaje doesn't sound good in Spain, but as somebody said "The world does not revolve around Spain", so I don't say Gabriel's translation is wrong. In any case, I'd translate it as follows:
Media del resultado clínico Promedio del resultado clínico
Mean: More accurately called the arithmetic mean, it is defined as the sum of scores divided by the number of scores. Or, put in other terms, the mean is the sum of measures observed divided by the number of observations. 68+99+78+84+100 = 429