It'll definitely be messed up, but not random, or at least not as much as with other ways. There's a significant percentage of the human factor, how does your hand move, which shapes will you think of, at which rate will you repeat the shame shapes etc. It's the same as the mouse detector RNG of bitaddress.org.

That's the best I can do is: Take few dices, put them on a cup, start mixing them, verify that they're fair, redo it to generate entropy for the seed.