Yes. Inefficient. As in less efficient than something else.
The crucial point is that generally it costs more in electricity than you will make in bitcoins.
But here it is not you paying for the electricity. It is Amazon! They let you have a tiny computer in the cloud on them!
You are going to make absolutely minute amounts of bitcoins from it. Probably not worth the effort of setting it up even after its been running a year!
The question was asked, I answered it.
*Edit* I worked it out. Its so slow that you'd be lucky to get a couple of pool shares a day. Really not worth the effort*
Really the only time when CPU mining can be of any use is when you have a lot of machines.
Usually that means botnet, but there are also the fair share of school admins etc that leave the school network mining away over night.
Not technically illegal unless their contracts prohibit using the machines for self gain, but I doubt you would keep your job if the boss found out!
the real question is the rate at which you can mine vs the instance cost. have you considered spot pricing?