No it is not your fault
And while you definitely have a point about the power creep thing, Jarl looked mediocre alongside several of the older legendaries aswell, not just the newest and hottest ones, Gorgotha fe. has been around for quite a while too.
Anyways i think people should appreciate how far away from being too strong Stonehammer actually is, and adding Impervious to his toolkit won't tip the scale.
I have met about 10-ish Stonehammers in pvp so far, i have seen him cast his spell twice, both times missing the extra turn, with no real Impact on the battle either.. 'ok all my stuff burns now, so what? I'ma continue destroying all your stuff now'.
I have skull-bashed my way through Stonehammers like i do regularily through Gorgothas cause i am too lazy to scout and it is really no biggie. If you want to make sure Gorgotha/Stonehammer teams don't slow you down too much, just don't go full skulltard and have some spelldamage alongside the skullspam in your team, the options for that are huge now.