Uh, balanced would be "good at everything". But not "great at everything". Mediocre isn't exactly what I'd describe Mario's overall stats (depending on the context0, and I still stand by that versatility can be an asset. For instance, Mario in Smash Bros. is categorized as an all-around close-range fighter but he has tools to take on most people and has a strong neutral because he has enough mobility, a good combo game, has good enough offstage, etc but not overwhelmingly great at combo, not great at power, doesn't have top-notch mobility, doesn't have best neutral, only decent offstage, and so on. Mario does have a weakness in being outranged and a bit of a predictable recovery BUT his lack of other glaring weaknesses makes him not as easily exploited.
This gives him a comfortable learning curve which helps people get successful without investing too much work, so it's a decent payoff and that's why he's popular in Smash 4 and Smash Ultimate competitive speaking, but he can pull his weight in a more casual format too.
Same for the Mario Baseball games and Mario Strikers Charged. I believe Mario's generally one of the stronger characters there because he works anywhere on the field, while batting, or on the bases while specialized characters cannot pull their weight (Bowser can't be fielding, Yoshi is not a good substitute for a tired pitcher, Toadette's batting is abysmal, Peach isn't good on the bases, and more). In Mario Strikers, Mario can work anywhere too but Hammer Bro. can't defend your goals and Dry Bones can't really do much without a glowing ball if you need to exploit an opening.
And there's Mario Golf where Mario has the balanced archetype that helps him in a pinch. He shoots straight (which works just about anywhere but might not be ideal for every spot compared to fade and draw), he shoots high (which goes over trees, again, helps anywhere but he's more affected by wind), and he shoots relatively far. Like, again, Mario works in a lot of spots and that can make him a pretty good pick for Mario Golf despite not really being exceptional anywhere.
So anyway, yeah Mario can be argued to be not a standout stat-wise and he can be standard, like a default option, but some people like games without perks or they like someone who can get the job done anywhere. I wouldn't agree with Mario being boring, though its understandable.