I'd agree it would've been better with the traditional RPG gameplay, but for me since the gameplay in the RPGs is a means to advance through a good story, and it's never been so bad it brings down the story imo, I'm fine with SPM and would prefer it any day over the games that don't bother having good gameplay OR a good story.
Another thing, Sticker Star's gameplay is arguably better or worse than SPM's, but I'd tolerate it just as much if it made up for it with a good story to keep me engaged like SPM did. If the game isn't gonna bother with a story, I'd rather it just be a main series game so I can run freely jump around a platforming environment, slow-paced games focused on NPC interaction and turn-based battles that don't have a story are really not my cup of tea. SPM may have mediocre gameplay but at least over 10 years later I can fondly look back on the characters & narrative that just really clicked with me and I still find myself drawing fan art of to this day. I can't say that for anything Paper Mario has touched since.
But you did remind me that once upon a year or so ago I wanted to aspire to port SPM to TTYD's engine since the assets are compatible, so we could experience what SPM would be like with the traditional Paper Mario battle system. Unfortunately reality hit that one hard, having compatible assets alone is hardly enough to make such a thing. Though there was a recent Blender importer for TTYD I saw footage of on Twitter that might remove some of the hurdles but that sort of project is still far off. Might be possible in PM64's engine with Star Rod but then the SPM assets would have to be rebuilt in the N64 style.