I feel like a lot of BS has built up around this entire line, and it's mostly my fault, so I'm going to try and cut through it to the heart of what I'm saying.
The good Mario RPGs have charming writing, fun side characters, and simple combat mechanics that are still detailed enough to be worth learning in detail. But there's a lot of other RPGs out there that are better than they are at those things. Deep storytelling? Pick a Final Fantasy starting from 6. Likable, three-dimensional characters? Tales. Deep, engaging battles? Shin Megami Tensei. All those things? Chrono Trigger. Just to give one example for each of the criteria that JRPGs are usually judged on, none of the Mario RPGs, even whichever one's counted as the best of the lot by any individual (for me it's Paper Mario 64), I'd say don't truly live up to the all-time greats of the genre - at least, not on their terms.
The appeal of the Mario RPGs is Mario, in every aspect. Not just his character and the strange and wonderful beings that he jumps on in the main games, or the world he inhabits, or the jumping, all of those things. And the best Mario RPGs lean into that, use all those aspects to their advantage. Super Mario RPG fundamentally doesn't achieve this to the degree that the later games do, and the biggest stumbling block is the implementation of the game's combat. He spends most turns throwing out punches and karate chops, and only saves jumping for special occasions; the iconic hammer from Donkey Kong is a regular weapon that effectively doesn't do much different, other than a bigger number, compared to those Bruce Lee moves or other weapons he picks up for their bigger numbers. They're there, but the way I see it, they're treated in a 'hey, this is a thing Mario does, right?' kind of way rather than made the core of his identity in battle.
That's where Paper Mario and TTYD succeed, that's where the Mario and Luigi games succeed, and that's where Super Mario RPG doesn't live up to its own title. So if it's not as good as other 'mainstream' RPGs, and it's not as Mario as other Mario RPGs, what's even left?
Ultimately, this is all just my stupid opinion, but I at least hope you can see where I'm coming from with this.