In OP's defense, though, he seems to be raising this concern in the context of overall poor/inadequate service. He didn't say "I had the best FA ever but he wore sneakers so I hate him!" It sounds like the sneakers could be viewed as part of a larger unfamiliarity with/lack of enthusiasm for the job.

Now me, though - once on a flight in OCTOBER I had an FA who was as nice and friendly as she could be, and took wonderful care of the whole cabin, but she was wearing white shoes (not sneakers, but flats, and not even particularly stylish ones at that)... just to be clear, this was after Labor Day. It was terribly upsetting, if my grandmother had been alive and traveling with me she would have fallen down dead.