These games strong point is IMO the worldbuilding they offer, and by skipping the first game you skip a large part of the reason I at least enjoyed them. Settings-wise, the stuff that is going on in Mankind Divided is a direct result of actions happening in Human Revolutions, and I would claim a large part of the enjoyment of the game comes from seeing how cause and effect plays out. You'll just be confused for different reasons. If you didn't play DXHR, you are gonna be utterly confused about who is what - but you will be if you played DXHR as well, so there is that. The main character is not quite sure about after what happened since the ending of the previous game, and that's a bit of a plot hook for DXMD to resolve.
Storywise, Deus Ex: Mankind Divided is a sequel that picks up half a year or so after Deus Ex: Human Revolutions.
Do I need to have played the last Deus Ex game to enjoy Deus Ex: Mankind Divided?Not strictly, but I would strongly recommend doing so.
