Personal quote at the beginning of a PhD thesis? Is it appropriate to put at the beginning of my PhD thesis a personal quote that very few people would understand ? Could it make my work seem not serious ?

More specifically, it would be lyrics from a song that reflects a personal problem , that my phd helped me to deal with eventually. There is no direct link with the actual content about my research. It just feels right to me to acknowledge what these 4 years of phd brought to my life, beyond pure scientific aspects.
 99% of the people reading my thesis would not understand the meaning, and it may even sound quite cryptic for them.
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.