Absolutely not! True, carrying extra weight will make certain poses more challenging, and there will be increased pressure on the joints in some poses. But not doing yoga until you lose weight is like not doing yoga until you get flexible. Among other things, we do yoga because of the tremendous physical and mental benefits it brings us in exactly those areas where we struggle. If we were already as strong and flexible and thin as we tell ourselves we should be, then we wouldn't have to do an asana practice at all. We could just sit around meditating all day until we found bliss.