It does look "retro" to some degree, the shading and colors are distributed in a similar way to what old hardware would allow, it's good to note not all pixel art is instantly retro, the term became a buzzword to the point it doesn't mean anything anymmore.
I can confirm the portrait art looks EXTREMELY tumblrish, just browse the art tag in tumblr and you'll see, personally I don't like it, but the person behind it can actually draw, unlike Undertale's author.
Undertale graphics aren't retro, they are just outdated, simplistic and minimalistic to the bone, I mean, if you go to the Atari era, you'll see a couple messy pixels with one color could represent a person, Atari games are great and fun to play that doesn't mean it looks good or the graphics age well.
Both games are considered retro, yet if you compare UT's sprites with these, you'll easily notice this is a superior technique, character design is superior too, UT's MC design reduces to "average person with shirt, pants and shoes", it looks directly inspired from a random dude on facebook.
/v/ used to love Yume Nikki, but after it got a Steam release they hate it, yes it's a hipster circlejerk.