Yes, as arguably the first "serious" Mario game (made by two very great developers) and just as arguably the precursor of the modern RPG, it's a great game, but I would also say it is "somewhat overrated". However, this is primarily due to Nintendo's refusal to recogize it exists, which--as is frequent in videogame culture (i.e. Earthbound)--causes it to become elevated in people's minds to a level usually reserved for lesser deities ("they-don't-ever-talk-about-it-so-that-must-mean-its-awesome" mentality).
In addition, I'd just like to say that Paper Mario was great as well (as someone once said, SMPRG was an RPG--Final Fantasy, really--with Mario characters, but Paper Mario was a real "Mario RPG"), as was TTYD, though I found the latter somewhat derivative at times (and many of the areas were too left-to-right). As for the "third" Mario RPG series, I consider Mario & Luigi, with its refreshing creativity, unique battle system, and mirth, one of my all-time favorite games. PiT, meanwhile, was cripplingly linear and relied too heavily on Bros. Items. Super Paper Mario was flawless in almost every way. That is all.