Do you have to be religious to be courteous, or can I just say "Next!"...?

It would help if you changed your attitude. When a person has God in their lives then that person sees things in a different light. There shouldn't be a judgmental attitude no matter if the person coming in is over weight. God sees our hearts and even the Christian's heart isn't always perfect. In our hearts we judge, but we should immediately turn from that judging and repent, so that we can be a good witness for God. Try changing your outlook on things and start enjoying things that seem so small and unimportant to you. When you start doing this then others see it and therefore appreciate you more as a person.