Depends. Sometimes all you can see is that the baby's mouth is attached. I've seen people where nothing at all showed, and then I've seen people who just rolled their tube top down to their bellybutton.
Honestly, I think that if *you* are uncomfortable feeding in public, you have every right not to. But immodest behavior, imo, should be far more up to the individual person.
There are certainly places I wouldn't want a baby being fed -- if it's not okay to feed the baby a bottle somewhere, it shouldn't be okay to feed them otherwise. And quite honestly, the toilet (which is often listed as where she should go to nurse) is someplace I wouldn't want to pop a bottle in baby's mouth.