You can delete your account in the mobile app: head to Profile > Settings (top right) > Delete account.
Danger! Please be aware that deleting your account will delete all your data and progress and make a running subscription you may have purchased through the Mimo website useless.
If you bought your subscription in Apple's App Store or the Google Play Store, it'll remain active despite deleting your Mimo account! In this case, please refer to How do I cancel my subscription?.