THD measurement is used to analyze AC signals. I think you dont need THD measurement because your output is DC. You just need to meter output voltage ripple. This measurement will show u quality of the SMPS output. Your output will be DC + AC(ripple). So your main signal is DC and some AC ripple. You dont need to meter THD of AC ripple. Just metering the varying DC output ripple will be both enough and professional ı think =)
Yea its not a technical term but you dont need to ridicule like that. Just answer the question. Its not so difficult to answer the question without regiment.