It's a pretty solid game heldback by some pretty subtle but significant flaws. I wouldn't call it mediocre.
I would say "solid with flaws" is roughly equivalent with mediocre. And I typically see people who haven't played more of the other games enjoy it more than people who have, which includes me, (and I'm not saying you haven't played, I'm making a general statement) it's just so lackluster compared to anything else, dungeons are nearly non-existent, smirk is garbage so the only pro to combat is that it's press turn, the story ranges from complex and somewhat subtle at times to an utter mess at separate points, the support cast are nonsensical at best, and let's be honest, they struck a bizarre balance in trying to be more user friendly where some staples of the series are flat out gone, while some things are vastly improved, the only exception off the top of my head being that they went back on Strange Journey's system for hunting fiends and made it the RNG garbage it was in SMT and SMTII
Those things, being a lot of the game, aside there is definitely good stuff, world traversal and locations are fine, a lot of side quests present actually interesting content if you're into mythology, and like all the Sandman-esque stuff with it, and aren't hampered down with all the writing problems of the main story, press turn is a good system though smirk is not, compendium is awesome and fusion, while there are some problems is pretty much the best it's ever been.
I posted the image for a reason, I'm not shitting on a game I dislike, I got every ending, did every sidequest and made every demon that wasn't Matador, the riders and Harlot. It's just that it doesn't have anything special against nearly any other game. It's fine, is it worth playing? Sure. Is it worth replaying? Meh, unless you're that in to getting the demons there isn't much going on. And sure, it's better than replaying the calendar games.
I have a lot of hopes for the follow-up game, and other than Lucy, Doi seems to be doing a good enough job blending the demons in, which is great, some of those designs are actually cool but they just didn't fit. New character designs are also at least visually interesting whether or not you like them, unlike IV's cast.