Hmm? Maybe setting mines inside the sumo arena alerts the town...
There must be a way! *also drops everything to play IVAN*
EDIT: No...
Setting traps angers the townsfolk, sadly. HOWEVER you can use teleportation to get outside the arena walls, and vomit everywhere or cover the floor with broken glass. That works just as well. You could also try spilling sulphuric acid everywhere, but breaking any wall results in the town getting pissed off, so that's incredibly difficult if you haven't got a companion to throw things at - and then you might miss and break a wall anyway.
Anyway, if you really wanted to see the spectators blow up you could just script in some big mines in the dungeon.dat file