This is in a DJ context, so I'm presuming you're talking about a DJ-style mixer with a crossfader, rather than a more traditional line mixer.
You've got at least two audio sources (at least one laptop, at least one iPad), so if you want to play them at the same time, you'll need some kind of mixer.
However, if you plan to play only one device at a time (presumably switching the cable from one to the other) then the only reason you'd need a mixer is if you want to use some specific feature of that mixer for your performance. You could, for example, put the laptop and the iPad on either side of the mixer, and alternate playing songs on each one while using the crossfader to blend/cut between them.
However, you may be using DJing software that is handling this, and if that's the case, then the mixing is already done in software and no hardware mixer is necessary.