Personal quote at the beginning of a PhD thesis?
Yes, it is appropriate and harmless. At worst, it will be ignored, and at best, it will serve to remind readers of your thesis that you are human, and maybe that they are, too.

More generally, academia.se seems to have many questions that follow the template "Is it appropriate to do [insert completely innocuous but slightly nonconformist action that isn't mindlessly emulating the way everybody else in academia does something]?" The answer to such questions is generally Yes. Moreover, for what it's worth, I personally have a favorable view of people who are not afraid to do things in a slightly unconventional way, and I'm sure there are others who take the same view as me.