They didn't earn those pokemon, so no, they do not deserve to.
It's like asking if people who roided up to get jacked deserve to have a spot at athletic competitions versus people who actually worked very hard to be at their level.
It's like asking if players with hacked gear deserve to be able to fight competitively against another player with legit gear, regardless of whether or not their gear are at the same tier. (Hacked gear but within game limits)
I don't play DOTA2, but I can use it as an example. Imagine grinding up for a Divine Rapier for whatever reason, going up to clash, then your opponent surprises you by having one himself because he hacked it in. Just scratch the why, semantics. It's a totally legit weapon, but the way they got it isn't. How would you feel going up against someone like that? What, they didn't wanna waste time grinding for it.
"It's just a game", some people say. But when you turn it into a competition, it's up to you if you wanna throw fairness out the window. Should apply to everything competitive, but it always made me wonder why this isn't the case for pokemon players.
I will humbly remember you about the competition basis: its about battling. You don't like other players to have maxed stats because you autoimposed some effort restrictions? Fine: ask for competitions with premade pokes. Competition is just about battling, not caring. Hackers gonna hack, even if you don't like it. Even through competition rules.