I kind of want to let that one scroll by on its own b/c it's mostly about game design and the like,
but I feel like I have some Thoughts on this that approach it from a different angle
I quietly feel like there's something else very fundamental that distinguishes sonic and mario
and it's that sonic and co are Characters
and mario and co are Mascots
there is, I feel like, a fundamental difference here. Mario doesn't have Thoughts or Opinions. He isn't going to beat up his enemies to a song that screams about how independent he is and how he hates cops or etc. Mario is a conceptual hole in the world that emits "Woo, Hoo, Wahoo" noises and the general hope is that you, too, feel "Wahoo" in response. Mario will never feel more-specific emotions that require you to empathise with him¹ or consider his perspective on things, because his goal is, as much as possible, to be an inoffensive contextless shape. Mario is merely the vehicle through which Mario Gameplay may manifest. He has no meaningful traits beyond that.
It is for this reason, I think, that when people call themselves Mario fans they are almost never referring to the character himself. There is nothing for fandom to seize onto there. He is a slippery surface upon which emotional attachment can gain no purchase. People instead tend to latch onto the gameplay, which, obviously, is generally considered to be very good and polished.
What's interesting about this is that I think it creates an almost completely opposite situation from Sonic, whose gameplay is ethereal and changes wildly from game to game. To be a Sonic Fan in the modern era is to either pine for a very specific set of 2-3 games - usually either Sonic 2-Knuckles or Adventure 1 and one or two other 3d games - or, more often, to be invested in Sonic as a setting and a set of characters and the gameplay is merely a facilitator for you spending time with that cast and that Energy.
I wouldn't think it would be especially difficult to be a Sonic fan who has never played a sonic game, given the almost completely unbroken chain of TV Adaptations and comics that the character has seen over the last few decades. And that's kind of the core thing - when the Mario movie was announced the big question I saw a lot of people asking was "what would it even be about?". No one has ever asked that question about a Sonic adaptation, because the answer is obvious. It's about sonic and co doing Sonic The Hedgehog Things, which is a concept that Sonic can have because the vast accumulation of media Sonic is in is not about Finding Ways to Express Sonic Gameplay - I've heard Sega is actively hostile to attempts to do this in non-games media - and instead is about Finding Ways To Express Sonic as a character, as a cast, as a world.
There's a world where Sonic media tried to be something more like Mario and simply throw 90% of everything out with every new iteration, but someone or someones at Sega are actively opposed to that idea and so every new form of Sonic media comes with at least a bit of the baggage of the old - even the relatively isolated Wii Black Knight/Thousand Rings games, explicitly written as one-offs, couldn't resist incorporating characters like Blaze who had very specific and loaded character arcs across previous work. And this stuff adds up. Every cameo, every glimpse of that broken moon from Sonic Adventure 2 in a later game.
Which all is to say, where Mario is intentionally smooth and ungraspable to make it easier to simply place him contextless into a new world, Sonic is pocked with the marks of a thousand different attempts at saying Something with him, at preserving his past iterations, at re-re-re-examining the increasingly growing cast of core characters. You could argue the results of this are incoherent, and perhaps they are, but this kind of fracturing is perhaps where fandoms thrive the most, because The Ability To Interpret is absolutely core to fan works. This is the kind of media-production style that creates thousands of tempting little spots where fans can build their own little headcanons, and I think this is core to why fans are so eager to play in that space. There's nothing more welcoming to fandom than a blank space that screams to be filled. And there's just so few of these in Mario - they're more like endless, forbidding chasms - and most of the ones that exist were created during the Gamecube era when, I assume, Miyamoto was busy in the bathroom for awhile and couldn't get out in time to stop the dev team from creating E. Gadd and Rosalina.
So this is all to say that, yes, I think gameplay definitely plays a role in the major differences between how sonic and mario express fan-creations and fangames, but I think The Goal Of This Character Existing might actually be the core expressive element that informs that factor - because those goals don't just inform narrative and characterization, but game design goals too, right?
Or at least that's what it seems like to me.
¹ The one exception to this I can think of is Fludd's death scene in Mario Sunshine, which is notable for being an incredibly out of place scene in an out of place game, as far as series tone goes.
