The Talmud (Megillah 15a) records an argument about what, exactly, Mordechai was calling out as he went through Shushan. One opinion there has it that he yelled out, “Haman is greater than Achashverosh” in order to arouse the king’s jealousy. The other opinion is that Mordechai yelled out, “The King above is greater than the king below” in a euphemistic fashion to imply that Achashverosh was attempting to usurp H-Shem’s Power.
Yalkut Shimoni writes that there is generally a difference between Jewish prayer and idolatrous prayer; whereas Chana’s prayer was quiet (Shmuel 1 1:13), Eisav’s prayer was a “great and bitter cry” (Bireishis 27:38). Like dogs that bark loudest when they have the least bite with which to threaten, an idol-worshiper’s prayer needs to be loud since it has the least spiritual power behind it.
Furthermore, Rav Eliyah Lopian suggests that, whereas physical people cry over physical phenomena, spiritual people cry about spiritual matters. Here, however, to counteract the possible spiritual effectiveness of Haman’s ancestor’s (Eisav) “great and bitter cry,” caused by the actions of Mordechai’s ancestor (Yaakov).
According to Yosek Lekach and the Vilna Gaon, Mordechai’s cry was inspired by his feeling responsible for the decree against the Jews. After all, his decision to refuse to bow to Haman, regardless of the logic, is what led directly to Haman’s anger with the Jews of Persia and beyond.
R’ Henach Leibowitz points out in his characteristic way that this should be a powerful lesson to us about how careful we must be to avoid hurting someone, even when we are in the right!
Taken as a unit, some commentators find great significance in the combination of these three motifs of the sackcloth, the city, and the crying. According to the Ginzei HaMelech, the loud voice represents Avraham because he spoke out powerfully against idolatry in a world filled with idols (see Rambam, Mishneh Torah, Hilchos Avoda Zara 1:3). The ash represents Yitzchak who allowed his father to symbolically sacrifice him. The sackcloth represents Yaakov, who mourned in sack upon being told of his son’s untimely death (Bireishis 37:33). Therefore, in a thoughtful, calculated action of spiritual symbolism, Mordechai used these to recall the merits of the forefathers, whose merits always protect their descendants.