Is it possible to have a user that's only capability is to work with Mosaico. Specifically to Create/modify templates
Unfortunately there is no single Mosaico privilege. You would achieve what you want using a combination of CMS permissions and CiviCRM ACL.
I would do the following:

Create a special account for this volunteer, that they will use just for creating the the templates.
Create ACL so that the volunteer account has access to only one group of contacts, a testing group, where you will have a single contact. This is just for them being able to select a group when they want to create a new mailing ( this stem is option, i.e. if you want them creating the mailing and preparing everything for you to review and then just send it to the real group if you are satisfied. (If you want them just to edit templates Mosaico then this step isn't necessary)
Subtract all the unnecessary CMS permissions. I am not sure which permissions would that be, since I don't have a WP installation (I am on Drupal). You will want that user to have:

backend access
access to uploaded files
CiviMail access



Additionally, you can use the navigation menu settings (Administer -&gt; Customize Data and Screens -&gt; Navigation Menu) to add a permission requirement for any menu entry, effectively hiding any option you wouldn't like them to see in the menus.