It's not like people would care at this rate, she practically uses a Kingdom Hearts Save Editor to give herself Ultima Weapon when she decides to play through the game's bosses on stream, and she's too insistent to use anything beyond Gardevior unless it goes down or she needs an HM Slave to get around the game.
When is generally consitered "Game-Breaking" 'Mons (Especally to the Somgon Turbospergs) tends to be due to a massive advantage that usually ROFL-stomps everything else or an easy to use strategy that can keep the opponent's team in check or outright bypass what said Opponent might have in the right circumstance. (Think Mega Rayquaza, the go-to example of being so overpowered just by being able to use it's Mega Evolultion alongside a held item or many of mons who had a significant presence in a specific generation.
That being said, Gardevior on it's own has been considered Rarely Used since Sun and Moon and was underused at best (it was in the Under Used Ban List (UUBL) in it's debut gen and was in Never Used in D/P & B/W) before then with the exception of X/Y where it's Mega was OU, but the base form was UU (Untill Sun/Moon where it's Mega was unceremoniously chucked into the UUBL and the base was promoted to RL.)
TL;DR: If Jerry were to try to play competitively with Gardevior, even with the other 'mons he'd use, he'll absolutly get ROFL-Stomped by anyone taking it even remotely seriously (even accounting how she'd raise them).