Most mitzvos are bitul torah, but you have to do them anyway. Speaking of mikra megillah, Mordechai was mevatel talmud torah to take care of tzorchei tzibbur, which is why he was only raztzui leROV echov and not kol echov, and also why he was demoted in the rankings between Ezra 2:2 and Nechemya 7:7. But he was mechuyav to do it.
Also, the comparison between leining and other mitzvos is imprecise, because leining is part of the mitzva of talmud torah, so it’s not bitul torah.