P5 sold a lot in the first month then dropped like a rock. You can tell that word of mouth wasn't good right from the start. People don't talk about it, and when they do, it's to drag it.
Compare that to games with good word of mouth like Splatoon, Xenoblade or even Persona 4.
Alright, since this sentiment has been expressed in a few threads on GAF, let's put that claim to the test. First, let's search for a game that had significant backlash against it, I'll go with
Final Fantasy XIII. You can notice the user score is much lower than the critics average,
which isn't the case for P5, who boasts a 9.0, putting it pretty close to its critical reception. I could bring up other sites with aggregate user scores such as GameFAQS (3.73[ PS3] vs 4.56), or MyVideoGameList which has them at 6.7 vs 9.4 respectively. In fact, you could also compare it to other critical/fan darlings such as Horizon, Nier: Automata, Nioh, and Breath of the Wild, and the only one who consistently ranked better than P5 on both critical and user reception was, unsurprisingly, Breath of the Wild. By all concrete metrics available, it's the second best received game of the year.
If we're going with more abstract measurements, then there's really no way to quantify this sort of stuff. There are going to be positive and negative posts about it, and that's that. I could tell you, from a purely anecdotal point of view, many people I know who tried the game loved it. The subreddit has a lot of activity, and there are plenty of fanart and cosplay to go around. So... is this about GAF, specifically? Well, you could argue the game has many critics here, but even then you also have many people who loved it,
some listing it as their favorite, others not. NeoGAF GOTY nominations remain to be seen, but I would be very surprised if it didn't rank highly there. Bad word of mouth is certainly not how I would describe P5's reception, but I rest my case.
EDIT: Also, for whatever that's worth, Yahtzee, who notoriously hates everything JRPG not called Paper Mario and Earthbound,
also liked the game. Dunkey expressed
similar thoughts. I'm only singling them out because I've seen other people express similar thoughts.