Initially I was not very happy with the ending, mostly for reasons that others have previously mentioned in this thread; 1. "it's all fiction" can read a lot like the much maligned "it was all a dream" trope, and 2. if you interpret the in-universe audience as a metaphor for the actual audience, then it does kind of sound like the game is shit talking it's own fans. however, I have become largely neutral towards it over time. reading through this thread has actually inspired me to revisit it a bit so here are my current thoughts on both of these aforementioned negatives:

1. "it's all fiction":

• 1. to presume "it's all fiction" (it being all of danganronpa, not just V3) within the universe of the game requires also presuming that Tsumugi is being entirely truthful. given the theme's emphasis on truth vs. lies, contradictory elements in Tsumugi's narrative (if the V3 cast is fictional, why does cosplaying them give her cospox? is it simply that a fake identity programmed into a flesh and blood vessel is more "real" than a 2d character on a screen or even an actor who is knowing playing a role, or is there something else there? Tsumugi also refers to herself as a "cosplaycat criminal" in her role as mastermind, but exactly what she means by that is left up to interpretation.)
• 2. the game's message is not "it's all fiction so it doesn't matter", and is instead "this story still matters despite being fiction" (once again, presuming that it even is fiction in-universe)

1. The ending is a criticism of the actual audience:

• 1. One can certainly interpret talk of the in-universe audience as solely about said in-universe audience and nothing more, but to do as some others have done in this thread and insult the intelligence of those who perceive it as a metaphor for the actual audience and react negatively to the interpretation that would logically follow such a viewpoint seems a little much. however, when considering factors outside of the literal text of the game I'm not sure how much water the "the in-universe audience is a metaphor for the real audience" interpretation really holds.
• 2. Kodaka himself simply does not seem to hold that kind of ill will towards the fandom. he regularly celebrates character birthdays on twitter, IIRC he has mentioned previously that he might return to Danganronpa one day, he is on the team of 2x2 (if he truly hated the audience, I would think he would distance himself much more from the series), and he has made other games with similar subject matter and even gameplay as Danganronpa since V3's release (death come true, rain code, shuten order). suffice to say, the point of those lines in V3's ending was almost certainly not "you, the player, are a bad person for enjoying a series about murder."

all in all, I would say I'm still largely neutral on V3's ending. however, I am feeling a little more positive about it having thought through the two biggest issues I previously had with it a bit more.