A couple weeks back, we learned that Super Mario Party wouldn't support the Switch Pro Controller. Not surprising, since Nintendo wants to keep everyone on a level playing field with controllers. Now today, we learn another tidbit that wasn't confirmed yet, but again, makes sense.
Super Mario Party is not going to support play in handheld mode. You can obviously play docked, as well as in tabletop mode, but handheld with the Joy-Con attached isn't going to work. With all the motion controls going on here, as well as the big focus on local multiplayer, hearing portable mode isn't supported shouldn't be too much of a shock.