I received mine too. Looking at the schematics https://github.com/96boards/96boards-sensors/raw/master/Sensors.pdf
The signal coming from the DragonBoard are level shifted to different level by the TXB0108 and TXS0108 (U4, U5) or BSS138 (Q2, Q3, Q4 , Q5).
So we can use them to interface most of electronic sensors which usually use 5 or 3.3 VDC.
You have to use the level shifted signal in the Mezzanine connectors. (G1, G2, G3, etc…)
For the UART, I think you will still need an adaptor like FTDI or else but you can link it directly to the Connector P2.
The signal 96Boards LS Connector from the DragonBoard and the Mezzanine does not change.
Hope my comment will help.