I will say again that I see people hiking in the Mojave (and elsewhere) in flip flops, bikini tops, shirtless (for men, all the time). I don't hike in a bikini (!) but I do hike in shorts and tank tops and make a major investment in sunscreen, as does everyone I know. I often wear a cap to shade my face, just as Barbara Thomas did.
It can be 115F in Los Angeles and people will be jogging dressed really scantily. While more humid than the Mojave, it's definitely part of SoCal culture to go to the beach, the local park - and yes, the desert, and wear as little clothing as possible. I've seen people hiking Grand Canyon, also, dressed in bikini tops and shorts (and it gets really hot at the bottom).
Not many stinging insects in Joshua Tree or the Mojave NP (scorpions and snakes, though, but covering up your body isn't going to help there). I've never seen a gnat or a mosquito either place. Simply too dry. If you start seeing flies/horse flies, then you're near some sort of water or something (grazing spot). But no one grazes anything in Mojave NP or nearby. Too dry. Many places truly are just dry sand.
Again, the heat in Mojave NP has been going up to around 100F each day, whereas in the place the Thomases live, it's been 116-118F. 100F is hot but, to me, it's not that hot. 118 is definitely hot. They are definitely part of a subculture of retirees living in Laughlin, Bullhead, Havazu, whose life revolves around adaptation to hot weather.