Kylo has zero poker face, and we only really see him snark when he has the mask on - I can see him flirting with Hux with the mask in place and avoiding taking it off not just because he's self conscious about his looks, but because otherwise Hux could see him staring, could see how he cringes when he makes a lame comeback, just generally see his big woobie younger-than-his-years face... it must be avoided at all costs...