The most important concept of maid cafe is "mutual confidence".
WRs (WaitResses=maids) trust customers and customers trust WRs.
WRs respect customers and customers respect WRs.
WRs love their shop and customers also love the shop.
Customers pay money, but it's not for foods. It's for the atmosphare and the circumstance WRs produce.

Maid Cafe is ultimate escapist place. Your everyday life may be tough, but when you are in Maid Cafe, you can, or should, forget all the troubles you have. WRs will do their best to relax you, with dreamy sweets and arguably the most loveliest uniform in the world, maid costume. So customers are expected to support WRs effort to create the relaxing atmosphare. What customers must not do is being insolently. It's true that the one who pay money is customer, but in a sense, the one who serve is also customer. That's the uniqueness of Maid Cafe.

I don't think being gaijin customer isn't any problem. But gaijin tend to chat in a loud voice that other customers feel annoying. Be a gentle "Master", and try to entertain WR a bit if you can. WR will be pleased, and their smile must be priceless.