Is it OK to use lip fillers 20 days after having jaw surgery and lips not swollen at all only some numbness?

I would recommend holding off. Depending on the type of jaw surgery you had, you still may be swollen at this time. At 20 days there still is a bit of residual swelling present. If you are determined to proceed with filler I would urge your practitioner to under, rather than overtreat. 

Wait until you are fully healed from your jaw surgery (swelling, numbness, open wounds, etc) prior to getting any fillers done. You don't want your body to be in an inflammation state at all at the time of fillers, if so, sometimes your body reacts to the filler in a way we don't like. Hope this helps!