Hector Berlioz Tells Us What's Wrong with Mozart

Yes, he does, on p. 65 of the Dover edition of his memoirs:

...it was owing to this cause that my admiration for Mozart was so lukewarm. Only Gluck and Spontini could excite me. And this was the reason for my coolness with regard to the composer of Don Giovanni. Don Giovanni and Figaro were the two of Mozart's works oftenest played in Paris; but they were always given in Italian, by Italians, at the Italian opera; and that alone was sufficient to prejudice me against them. Their great defect in my eyes was that they seemed to belong to the ultramontane school. Another and more legitimate objection was a passage in the part of Donna Anna which shocked me greatly, where Mozart has inserted a wretched vocalise which is a perfect blot on his brilliant work. It occurs int he allegro of the soprano aria in the second act, "Non mi dir," a song of intense sadness, which all the poetry of love finds vent in lamentation and tears, and which is yet made to wind up with such a ridculous, unseemly phrase that one wonders how the same man could have written both. Donna Anna seems suddenly to have dried her tears and broken out into coarse buffoonery....I found it difficult to forgive Mozart for this enormity. Now I feel that I would shed my blood if I could htereby erase that shameful page and others of the same kind which disfigure some of his work.(1)

(1) Even the spithet "shameful" scarcely seems to me strong enough to blast this passage. Mozart has there committed one of the most flagrant crimes recorded in the history of art against passion, feeling, good taste, and good sense.

Given in Italian by Italians! You don't say!