I can't imagine this would be an issue, but is there any reason why I can't/shoudln't install OSX 10.11 Yosemite on my MacBook Pro 2016? Sierra has always felt very sluggish to me, and it is the first OSX release where apps consistently crash. El Capitan was much more solid in my opinion and I would like to downgrade. Is there any reason why I shouldn't?
No you can't install El Capitan.
The firmware won't allow any version of macOS earlier than macOS Sierra 10.12.1 (16B2657) to be installed.
Also, for future reference, if this model ever needs a firmware update, it is possible that the earliest version of macOS you will be able to install in future will be a version later than macOS 10.12.1.