Does a monster's level affect drop rate?

You should take the maps/spawn of the monsters into account as well. Going with your example, even if there was a slightly higher chance for macis to drop crash, the spawn rate is terrible, whereas the officer skeleton map has a decent spawn, which would probably increase the chance of getting it in the long run.

Since we don't know much about drop rates in maple, i usually go with the smallest map or the monster with higher/faster spawn rate, and of course monsters that have their own map as opposed to others that share it with 1 or 2 other types of monster.