All you'd have to do, is disabling Notes for your Yahoo account. Go to Settings - Mail - Accounts. Select your Yahoo account, and slide Notes to the left, to disable it.
If you have notes saved in your Yahoo account, and want to keep them on the iPad, move them to your iCloud account before disabling Notes in your settings:
Open the Notes app, select your Yahoo account(1). Then tap "Edit"(2):
Then either select the nptes you'd like to keep/move, or choose "Move All" at the bottom:
Now select the notes in your iCloud account, or create a new folder.
Done! You'll notice immediately that the notes you selected in the Yahoo account will disappear from there, and the number of notes in iCloud will increase.