An original iPad cannot play this app, based on the iPadOS 15 requirements. Among others, iPad Air 1, iPad 4th and earlier, etc., cannot, per the published specs.
As for the crashes, the folks that know best are the game developers.
Given the supported hardware list at the developer website, support testing appears incomplete.
I’d guess the game developers are using one or more cross-platform frameworks for the game too (possibly Unity?), and frameworks which can sometimes have subtle and difficult-to-debug framework errors. Not only is the game running, but what is often a very complex and frequently-updated cross-layer from a third-party vendor is also involved. And these cross-platform layers can sometimes get tripped by even subtle device differences (which graphics, how much memory, timing, etc) and device vendor operating system updates among the various platforms that these cross-platform layers support. But this is in general, and I’m not familiar with the internals of the MTG: Arena game.
There are also apparently multiple third-party products parsing the log files for this game, and using that to control and play the game — there does not appear to be a formal API for hooking your favorite AI into this game — which means bots, bot-speed activities, and bot-produced game server loads and sorts of automated activities, and most (all?) of these seemingly parsing lots and lots of log text as input. This is not optimal for low loads, and for stable servers and stable gameplay.
A place where there are more discussions than here in ASC: https://www.reddit.com/r/MagicArena/
Another potential cause of problems for iOS and iPadOS apps can be add-on VPN apps, as some of these can potentially cause issues when purportedly disabled. Remove all, restart, and test again.
TL;DR; game crashes are best get discussed with game developers.