Can Chinese citizens apply for a DTV visa for Muay Thai training in Thailand?

You can but get ready for some serious scrutiny. Sometimes they reject you despite of perfect and authentic documents simply because your fellow Chinese have been doing some terrible things here abusing various types of visas. The best thing you can do is to apply in China. Other places may reject you because of distrust. No matter how good your documents are, if they simply just won't trust them simply because of the past committed frauds by so many Chinese, it's fruitless. If your documents are real, apply in China since they will find a way to authenticate. Embassies in third countries are unfamiliar with Chinese documents like bank statements and they may rather simply reject you than trying to figure out of the documents are real.