There is nothing wrong with the grammar of"What is your weight?", abdu-ki.
Remember, however, that many people don't like nosy personal questions. If I didn't know you and you asked me "What is your weight?", I'd probably tell you. But people who are sensitive about their weight could easily take offense if you asked that question. If you are not worried about offending some listener with personal questions, then you have no reason to avoid asking them.