fwiw I think pink hair should be perfectly fine but there are some rules with dress that are a good idea. I don’t care if you wear a dress, slacks or have rainbow hair but generally I prefer people I do business with to wear stuff.
Then there’s food safety. Things like hair nets and prohibiting certain jewelry and outfits in food processing plants is another example of when sometimes it’s ok to limit personal expression for the sake of others’. A server though? No such reason. Maybe a hair net if they brought food to your table or were also a chef.