The Crown of Thorns was originally placed on the head of Christ as a form of mockery and has come to represent the suffering associated with the crucifixion. This is an old symbol, however, and some people choose it for their body art simply to represent trials, tribulations, and their own suffering.
It doesn’t matter whether you are religious or not with a symbol this old – it’s all about context. So, whatever you place with it and where you decide to put it are going to ultimately determine its meaning.