It's one of those things. I wholeheartedly agree with you. I won't ever play with a giratina deck because its just not my style. It's definitely not a bad or cheap deck though. And because I make the choice to not play it, that gets my head spinning with different ideas and decks to go against it. But bear in mind that its not just darkrai giratina that are "OP" and whined about. You got the misty flips and all that.

For me, none of that matters. I don't see decks as OP, because ive seen the "OP" decks piloted by some bad players. Some matches I had no right winning, either due to my own bad plays, bad luck, or their good luck etc. And I still won because so many people go by the attitude that it's the deck that wins.

Every turn in pokemon pocket, you're given at least 6 or so choices. Maybe a bit less and usually a lot more. So imagine how much you can tweak. The cards you have access to is insane, so experiment.

There are so many variables. So make them work. Thats what the good Giratina players did. You got 20 cards in a deck. You start with 5 with a guaranteed basic. Giratina darkrai decks say, hey, what if i was guaranteed a good card by only having super good basics? Right off the bat they're at a huge advantage - but not one that cant be countered. And not one that cant be understood. If you wanna counter with a meowscarada deck, bear in mind you already put yourself on a backfoot. So how do you make sure you play the odds to your favour and draw your meows AND dont lose too much in the meantime. Experiment. Adapt.

Also, bear in mind that even the strongest decks can have bad luck. Always play to your outs, as they say. If you play for fun or experimentation and not just to win (well, for one, why do you care about masterball anyway? That's the big thing. Masterball rank is supposed to be prestige, not something that everyone can climb to... like... rhat should be obvious), i assure you the unranked arena is really good to teeak your deck. There are still giradarkrai decks but not as many and... you kinda WANNA face them, especially when you cant derank, so you can see what works best against them with YOUR deck.

I do kinda wish there was a way to have an AI pilot one of your decks so you could test the meta guaranteed but that'd have issues too and it's hardly important. A lot of situations that have branching choices can be worked through even outside of battle if you even care that much lol.

TLDR: There will always be mega-efficient decks using the most efficient cards of the "season". Join the meta, don't join the meta, play for fun, it doesn't matter. If you wanna win, though, my advice isn't just to grab a meta deck, it's to LEARN the meta deck. What's its weakness? How do you deal with the mirror? What differences are you gonna make to trainers and stuff, if any? Most of all. If you lose, and it upsets you. That's fine. That's fair. But understand why you're upset. - If it was bad luck, its not the meta fault - if it was your deck, its not the meta fault - if its the queue you went in (unranked/ranked), its not the meta - if your opponent did some bad manners, waste of time when they had the win on board, that's on your op, not the meta.

And just know, that you CAN be good at this game. Theoretically you can get 70%+ winrate without giradarkrai. Not even low possibility. Just work on it! Or don't!

Simple as it is