I'm usually the most casually dressed person in J or F and often the youngest as well. FAs still treat me wonderfully. I do get looks sometimes from other passengers as I walk by them in line, or when they walk through the J cabin on their way to Y.
My travel outfit usually consists of sneakers (latest travel sneakers have been Adidas NMDs I picked up in Lisbon), comfy joggers, a t-shirt and a light jacket.
If I know I'm going to see some of the higher ups in my company on my flight, I'll usually dress up a bit. I once got to the airport in GVA and realized the CEO of my company was on the same flight back to LAX. He didn't even give me a second look, despite having met me several times...