MF isn't a good crit ADC because she is 1) vulnerable and immobile 2) bursty (as opposed to dps oriented), has 3) mediocre dps, 4) mediocre target access.
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vulnerable and immobile > will have a hard time staying alive in a prolonged fight to output consistent damage per second with auto attacks > low incentive to build Critical Chance and Attack Speed
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bursty > most of her damage comes from spells which scale extremely well with AD and Armor Penetration > low incentive to build Critical Chance and Attack Speed
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mediocre dps > no good steroid (her W is meh), low base AD > low incentive to build Critical Chance and Attack Speed
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mediocre target access > mediocre range (550) and no mobility, making it difficult to reach/kill priority targets in a fight or kite with auto attacks > low incentive to build Critical Chance and Attack Speed
As long as none of the above points are addressed, she will never be a good crit ADC, which is fine by me, I like her the way she is now, not every champion needs to be identical.