The notion that people should dress up for upper-class travel is largely a thing of the past. What's acceptable nowadays is basically what's acceptable in any store or restaurant:
- Cover your genitals
- Cover your abdomen
- Cover your behind
- Wear something under your feet
- Don't be noticeably dirty or smelly.
I choose to go well beyond this in how I dress myself most of the time, but I am not offended by someone who merely meets these minimums. For example, shorts and a t-shirt with sandals are fine-- assuming the clothes
fit!