Gardevoir has always been good and I would argue that Metagross always was too (though that last bit isn't popular opinion). Sableye was super stronk before because it was bulky, had Prankster, and had access to both reliable recovery and status moves on top of no weaknesses. It could stop damn near any physical attacker and even many special attackers. Gardevoir was good with it's relatively high SpATK, people would often put a Choice Scarf on it and make it a revenge killer. Mega Gardevoir is REALLY good (though I don't see people use it too often...) and can put in so much work if the right plays are made. Metagross was always a good Pokémon but the problem was that there were other Pokémon that could do what it did but better. Mega Metagross, however, has so far been proven to be undeniably badass and versatile. I have two of them, in fact. My favorite Mega Metagross is my special attacking one because nobody ever expects it. Though people also don't expect my non-Mega Metagross, so that's always a plus now.

tl;dr: Yes, they are both good Pokémon and can be good without being Mega. Though also very good when Mega.