I believe it's a title rather than a mantle. There's no evidence to suggest that being labeled the Merlin provides any kind of power boost. Plus they're elected, not selected, which is kinda not how we've shown Mantles to really work.
On the flip side, in order to be chosen to be the Merlin, you're probably already the most capable wizard alive at the time, so I don't think a mantle would even be necessary.