Worry about looking bad when you are out with friends, on a date, or trying to attract guys. At work, worry that you are doing your job well. As far as the dress code goes, it is not your restaurant. It is easy to accept work rules if you understand this. And to remind ourselves that if we want to dress the way we want, then we have to open our own restaurant. In the meantime, if you feel too uncomfortable tucking in your shirt, either quit your job or revise your body image attitudes. Don't mean to sound harsh .. .I have body image issues too .. but I don't take them into my workplace.
I would suggest blousing your shirt (pull a little of your shirt out of your pants). Make sure your pants are a little baggy and make sure they are not super low rise.