You can display the Mii associated with either Miitomo or your Nintendo Network ID. Please note that your Nintendo Account must be linked with Miitomo and/or your Nintendo Network ID for this to work.

To display your Miitomo Mii:

In Miitomo, open the Menu from the bottom right of the main screen, then select "My Nintendo Missions". From here, link Miitomo to your Nintendo Account and your Miitomo Mii will be displayed on My Nintendo.

To display your Nintendo Network ID Mii:

From the Linked Accounts page, link your Nintendo Network ID. Your NNID Mii will be displayed on My Nintendo.