Yes, you can bring a Mii if you wish.
http://www.nintendoworldreport.com/news/60055/new-switch-sports-overview-trailer-outlines-features-and-customization
Some new details have emerged for Nintendo Switch Sports in a new trailer.
Among the items shown include new weapon types for chambara ("light swords" and twin swords), obstacles in bowling including sliding walls and ramps, and a "shootout" soccer mode that will be the first to use the Ring Fit Adventure leg strap. Character customization will be available in addition to the option to use an existing Mii.
Online play will include ranks that improve with wins including a "Pro" tier, as well as points that accrue toward unlocking additional customization options.