I've been complaining since day 1 of Smash 4's reveal that they should've just ditched the infamous Brawl scowl in Smash 4.
Anyway, you can technically make this argument for Mario Kart's Mario and some other games but the difference is that Mario makes cheerful noises and sometimes does make a cheerful face when he's boosting or making a stunt. But yeah, you need to mention the voice samples Mario makes, it's another remarkable contrast between Smash Bros. and the Mario games. Kind of how Donkey Kong, Bowser, and Diddy Kong have really strange noises. The reasoning behind these "realistic" noises is because "realism" but that's a terrible reason, akin to giving King Dedede penguin noises or Greninja frog noises or something.
Finally, although this is a Brawl thing, don't you think it's strange that Mario has gotten a post-apocalyptic wasteland for a stage as well? It's like a perfect compliment for Mario, lol. We also used to have Luigi being this angsty thing but that's been reduced, thankfully, even Sakurai wanted Luigi to be "everyone's favorite man in green" but Luigi could've benefited from new and cheerful voice samples.
Either way, I hope the developers change this or at least make it look like Melee or Smash 64's facial expressions because this is really out of character for Mario. It's almost like someone should tell him that he should get out and face the sunshine or smell some roses when he's supposed to be doing this on a daily basis.
Also, look at the freaking amiibo.