Super cute - I love DIY gifts, I always find them so much nicer and more personal than store bought things. Also, it's so unique that you're not likely to have the same thing!
I think it would be cool to also perhaps put the dry ingredients for the mug cake in a mason jar or something to give with the mug, so that way the recipient could just put all the dry ingredients in the mug, add their own wet ingredients as required and then the mug cake is ready to go! Yum!