Does Undertale have flaws? Yes. Sprite quality in inconsistent (compare Asgore/Toriel to Frisk), animation of sprites is pretty weak even compared to 8-16 bit games, the battle style is meh for some, and the game can seem very heavy handed in it's message on non-violence. But please understand, every game has flaws to some extent.

Is it the best indie game ever? That is subjective. FNAF and its sequels had similar acclaim when they came out. Who knows? Maybe next year an even better indie game will come out.

Is Undertale a shit game? No. Even if you dislike the art style, battles or even the story, you can't deny that it has already made a huge impact online and generated a lot of praise from fans and critics. The music and story are being praised, and the character dialogue is well-written and fresh.

Complaints about Tumblrism:
1. The game treats your character like a special snowflake. This is true with almost all games though. The player character defies all odds and saves the world/princess/etc. It's kinda a trope.

2. SJW agenda being shoved around? Okay, so there are two homosexual relationships in the game. Yes, but the game treats them as just relationships. The fact that the individuals are of the same sex is never brought up. I could easily re-write either Undyne/Alphys as a male or the bunny guard of the two Royal Guards as female and it wouldn't really change much.

Complaints about Furries:
1. They're monsters. What do you expect? There are also undead monsters, but there are no claims about necrophilia. Plus, the one true furry, So Sorry?, is treated like crap both in game and by the fanbase.