It's not like they contrast too much, really. I don't know, I just wasn't outraged [no pun intended] when I learned Garchomp was her signature Pokemon.
Yeah, it is sort of odd for someone as serene as Cynthia to have something as beastly as Garchomp, but I never really questioned it, to be honest. Also, you typed Metagross as 'Meatgross', lol.
I know...but it feels so unnatural, compared to the rest. Lance has Dragonite, Steven has Meatgross, Red has Pikachu/starter, and Blue has Starter. Also Alder have Volcarona. But they all fit!
It was always kind of implied that Cynthia's signature Pokemon was Garchomp since it's her most powerful Pokemon and has made many appearances alongside her in the anime, manga, etc, if that's what you're asking.