As several people have noted, currently there isn’t a toggle to switch the “Do Not Disturb is On” notification on or off. It’s a new feature that Apple released with iOS 12 and if you’d like to have them address it you should copy the message below and add feedback at: https://www.apple.com/feedback/
“Please offer a toggle in the settings for Do Not Disturb to switch the “Do Not Disturb is Turned On” notification on and off. I, like many, have set DND on a specific schedule and am fully aware when it is on or off. I understand that some users may accidentally turn DND on and would prefer a notification, but the constant reminder and need to dismiss the pop-up notification detracts from the iPhone experience and aesthetic for those of us who are aware of when we’ve scheduled Do Not Disturb. Thank you!”