Liking some customers and disliking others is human. There is no reason to worry about that. However, you have to be worried about not letting them know. If you show your primary emotions to your customers, the ones you like might sense that you are only interested in their money. The ones you dislike might sense that you despise them. Neither is good for business. Manage to always bring the best version of yourself to work. As the Chinese saying states: the day you cannot smile, keep your shop closed.