Can customers see when my restaurant is online on WhatsApp?

Only if you let them. In privacy settings you choose who sees your last seen and online: Everyone, My contacts, My contacts except, or Nobody.

• Options: Everyone, My contacts, My contacts except, or Nobody.
• Trade-off: if you do not share your own last seen or online, you cannot see other people's.
• Being online does not necessarily mean a customer has read your message.
• A blocked user loses access to your last seen and online status.