Q. Is one permitted to receive an aliyah if one cannot (does not know how to) read along with the baal koreh?

A. Remo (O.H. 139: 3) disagrees with the Mechaber and rules like the Maharil that nowadays we may give an aliya to a blind person (see prior question) as we give an am haaretz (an ignoramus.) Mishna Berura (ibid. 13) explains that although he does not know how to read, he still complies because of “shomea keone” (listening is the same as saying.) Biur Halacha (beginning of s. 141) further elucidates that this is a special dispensation for the blind and the uneducated, to avoid public embarrassment and for maintaining the peace.
Piskey Teshuvos (ibid. n. 31) discusses and quotes opinions as to how an ignoramus that does not understand the Torah words being read and does not even comprehend the meaning of the brocho he recited from a transliterated Hebrew card, can fulfil the minimum requirements for an aliya. Therefore Poskim recommend to call an am ha’aretz only for the hosafot or extra aliyos given on Shabbos.
Horav Shlomo Miller’s Shlit”a opinion is that if he believes that Torah is G-d given, in need you may give him a regular aliya.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a