A quick Google search reveals that we're not the first to discover this, but we thought it was worth sharing. If you're interested in trying it, go into the International keyboard settings in the General section of your iPhone or iPod touch Settings and select any of the Chinese keyboards in addition to your current keyboard.
Now, go into an existing note or create a new one in the Notes application. Switch the keyboard to the Chinese keyboard using the globe icon to the left of the spacebar and enter a single character. Backspace it and switch back to your native keyboard. Presto, the note should be free of Marker Felt. Saving the note will reveal that the title in the list has also undergone the same font change, and future edits will maintain it.
Adding new notes will require this little tweak every time, so it's not an all-around solution to the lack of font selection in Notes. If you're really, truly bothered by Marker Felt, however, it's a step in the right direction.