i have asked similar questions about showing certain bodyparts in the past and most posters curtly told me to just do it, that nobody is looking and no one cares.
But WE care, and I understand.
I think there is truth in this though, to a large degree, no one does care, and I strongly doubt it is something anyone else would notice or think about. We are used to seeing people in summer, of all shapes and sizes, going about their business, and we are generally too caught up in our own little world's to notice someone's upper arms. I know I don't!
I like to people watch, from cafe's train station benches, Booths restaurant window, haha, i love seeing what people wear, their haircuts and so on. But I never, ever judge people - the more variation and individuality the better! I have never focused on a specific body part and have admired people of all sizes and ages.
Anyone who would judge or look at a jiggly arm negatively isn't worth worrying about, they will be vanishingly rare (no matter what crap they talk on the internet) or otherwise a bit unhappy.
Please free yourself from the fear of liberation in the heat. Enjoy your body and what your arms can do. I used to hate my body when young, and now i am nearly 50 I am making up for it by loving the shit out of it whatever it does. It's mine, and I owe no one toned extremities 