Not necessarily. You can definitely mix just using the iPads. I haven't closely checked (maybe someone who uses iPads to mix can confirm) to see if the iPad app is apples-to-apples on par with the computer software mixer control. My recollection is that there are a few features (not necessarily very essential ones, though) that weren't included in the iPad app, but I could be wrong.
In any event, personally, I like to set up my XR18 for gigs using my laptop, at home or at my leisure, since it does carry the most recent updates and all of the control/interface features, and then save this layout. (You can even connect the iPads at the same time to see how all of this interacts and syncs up.) Then when you first connect using the iPad, it should ask you to sync to the XR18 (rather than sync to the settings as they are currently on the iPad). But none of this precludes you from doing your setups on the iPad exclusively.
Because I always insist on having redundant backups on gigs (the show MUST go on, you know!), I carry a laptop (Lenovo 2-in-1 convertible touchscreen tablet/laptop) that includes a network port with a hard-wired connection to my wireless router. This way, if I find that the wireless connectivity is dodgy in the venue, I can failsafe using wired laptop. I keep this on stage at a handy spot for us as an additional mix location. My theory is that the one time you don't have a connected backup plan will be the one time you need it (Murphy's Law).
If you know how to properly set up a router to avoid or eliminate wifi traffic from competing with your devices, you should be good to go 99.9% of the time. Wifi traffic is an issue regardless of what digital mixer you're using under control of wireless devices, and it's discussed quite often on this forum. If you're not well-versed, I'd suggest you read up.