I was looking through badges, and I've noticed that those three actually have a really high threshold to be reached in order to be awarded (compared to the difficulty of the task itself), proven by the fact that only 391 users have been awarded Mortarboard, only 5 have been awarded Epic and none has ever been awared Legendary. Perhaps an escalation along the lines of Popular Question / Notable Question / Famous Question like 1 (Mortarboard) - 25 (Epic) - 100 (Legenday) would fit better?
No, I don't think they are too high. Each SE site has its own voting culture, and it's been noted time and again that we don't vote much - we vote less than, for example, Unix & Linux. We have only two 100k users, U&L have two of those and one 200k user. They also have awarded the Legendary badge thrice. Yet we best them on all but one of the traffic stats. We are just a tough community.