The Talmud tells the following story:
Eruvin 54b
רבי פרידא הוה ליה ההוא תלמידא דהוה תני ליה ארבע מאה זימני וגמר יומא חד בעיוה למלתא דמצוה תנא ליה ולא גמר א"ל האידנא מאי שנא א"ל מדההיא שעתא דא"ל למר איכא מילתא דמצוה אסחאי לדעתאי וכל שעתא אמינא השתא קאי מר השתא קאי מר א"ל הב דעתיך ואתני ליך הדר תנא ליה ד' מאה זימני [אחריני] נפקא בת קלא וא"ל ניחא ליך דליספו לך ד' מאה שני או דתיזכו את ודרך לעלמא דאתי אמר דניזכו אנא ודריי לעלמא דאתי אמר להן הקב"ה תנו לו זו וזו
R. Pereda had a pupil whom he taught his lesson four hundred times before the latter could master
it. On a certain day having been requested to attend to a religious matter he taught him as usual but
the pupil could not master the subject. ‘What’, the Master asked: ‘is the matter to-day?’ — ‘From
the moment’, the other replied. ‘the Master was told that there was a religious matter to be attended
to I could not concentrate my thoughts, for at every moment I imagined, now the Master will get
up or now the Master will get up’. ‘Give me your attention’, the Master said, ‘and I will teach you
again’, and so he taught him another four hundred times. A bath kol issued forth asking him, ‘Do
you prefer that four hundred years shall be added to your life or that you and your generation shall be
privileged to have a share in the world to come?’ — ‘That’, he replied. ‘I and my generation shall be
privileged to have a share in the world to come’. ‘Give him both’, said the Holy One, blessed be He. (Soncino translation)
It seems like the reason for this student's struggles could have been that he had below average intelligence. If the student was considered (either at the time, or later) an amora, then this could potentially be an example of one who had below average intelligence.