There are only a few situations where they matter:
HP
Ditto copies most of the target's stats, but still retains its own innate HP stat. Thus, you'll want to have that stat as high as possible, including maxing out the EVs, except that you could choose to leave it one point away from maximum if you figure the balance of even/odd numbers benefits it to do so in the presence of possible factors like Ghost Curse, Stealth Rock, or poison damage. In the event that Ditto transforms into something that doesn't have HP EVs, it effectively gets to benefit from EVs in three stats at once: its own HP EVs, and whatever two stats the opponent chose to focus on.
Hidden Power
In games where Hidden Power exists, if Ditto transforms into an opponent that has that move, the opponent's Hidden Power type will be determined by their IVs, but the type when Ditto uses the move will be determined by its own innate IVs instead, which can be different. For example, if you expect you could be transforming into a Jolteon that uses Hidden Power for Ice, or a Magnezone that uses it for Fire, what you could do is have your own IVs configured to produce Ground type, and be able to hit both of them super effectively. On the other hand, if the Hidden Power users you expect to see are Venusaur (who normally uses it for Fire, often with a mind to taking advantage of sun) or Landorus (who mainly uses Ice just to have something productive to do in a mirror match) you might want to alter your approach around that.
Speed
This doesn't come up very often, but it can be vitally important when it does. If both players send out a Ditto at the start of the battle, or if one of them used Explosion to KO both Pokemon and they both send out Ditto after that, the game has to determine some order for whose Imposter fires off first, and the system they choose to use is simple speed order. (This speed order applies to all abilities that fire as soon as the Pokemon shows up, with the exceptions of Neutralizing Gas, Unnerve, and As One, which are "faster" than all other entrance abilities.) While people normally think of winning a speed race as a good thing, it's not always everything it's cracked up to be, and so it is here. By "winning" the speed race, your Ditto will transform into the other Ditto, which doesn't generally accomplish a whole lot...except that it means the PP supply of your own Transform will drop from 10 or 16, down to just 5. Meanwhile, once one Pokemon has successfully transformed, any further attempt to transform between the same two Pokemon (no matter who's doing it) will fail, so the slower Ditto will not end up transforming at all, and will keep its full supply of Transform PP. In the rare case of a double KO at 2v2, where Ditto was the only option left in the resulting 1v1, this single speed check proves decisive, as neither side now has anything left to switch to, and have no choice but to keep spamming Transform back and forth, which of course fails on both sides. The Ditto that first transformed will then run out of PP first, and be forced to use Struggle: since Ditto's attack stat is too low to 4HKO itself, this results in dealing more damage to itself than it does to the opponent, and the Ditto that "succeeded" in transforming will ultimately lose the battle. Mainly for this reason, competitive Dittos are usually recommended to have their innate speed as slow as possible, including a 0 IV and a negative nature, to ensure that they will never win a "Transform war" like this unless the opponent's speed is also as slow as possible, in which case it can't help but come down to a speed tie.
Another factor where it can matter, and also one where it benefits Ditto to have a low speed, is if Ditto and a Pokemon with Intimidate both come out at the same time. If Ditto ends up being faster in this case, Imposter will fire first, then the copied version of Intimidate that Ditto acquires in the process, then the other Pokemon's Intimidate, and both of them will end up at -1 Attack. However, if the Intimidate user is faster, Intimidate will fire first and apply the attack drop while Ditto is still its original self. Then, when Imposter fires, part of the transformation process involves throwing away any stat changes it used to have, and copying the target's stat changes (which, in this case, are all 0s). The copied Intimidate then happens last, with the net effect that the other Pokemon will end up at -1 Attack, but Ditto will be unhindered, at -0. Similarly, if Ditto is lucky enough to be able to transform into Zacian, it's a good thing that their Intrepid Sword will fire first, then Ditto can transform into them to copy their current stat stage of +1 before getting another trigger from newly acquiring Intrepid Sword, which will cause Ditto to end up at +2 Attack (and usually with a Choice Scarf speed bonus to boot).
