It's not an indie game, it was indeed developed by Ubisoft! It was a much smaller project than their main game releases, though, and wasn't as heavily marketed.
I can't say with 100% certainty why it ended up successful, but I would imagine it to be thanks primarily to the atmosphere and gameplay.
The combination of a platformer and turn-based battles is part of why the gameplay is so intriguing to me. I could explain how it works exactly, but I think it might be better if you see for yourself! The game isn't particularly expensive, either.
EDIT: After looking through the other comments here, I have a feeling you might be underestimating the appeal of atmosphere. People play plenty of games that don't have top-tier graphics, because amazing graphics aren't the point of those games; the graphics they do have serve to build the game's atmosphere.
In the case of Child of Light, the graphical style is reminiscent of illustrations in childrens' books, giving the game a fairy-tale feel and making the player more immersed in the perspective of the child protagonist.
Undertale is, in many ways, a parody of video games in general, and the retro-style graphics reinforce that.
I would imagine that, in most cases, the graphical style in games has been deliberately chosen. Of course, less involved graphics also cut down on development time and cost, allowing the developers to focus on other aspects of the game.
I can't say with 100% certainty why it ended up successful, but I would imagine it to be thanks primarily to the atmosphere and gameplay.
The combination of a platformer and turn-based battles is part of why the gameplay is so intriguing to me. I could explain how it works exactly, but I think it might be better if you see for yourself! The game isn't particularly expensive, either.
EDIT: After looking through the other comments here, I have a feeling you might be underestimating the appeal of atmosphere. People play plenty of games that don't have top-tier graphics, because amazing graphics aren't the point of those games; the graphics they do have serve to build the game's atmosphere.
In the case of Child of Light, the graphical style is reminiscent of illustrations in childrens' books, giving the game a fairy-tale feel and making the player more immersed in the perspective of the child protagonist.
Undertale is, in many ways, a parody of video games in general, and the retro-style graphics reinforce that.
I would imagine that, in most cases, the graphical style in games has been deliberately chosen. Of course, less involved graphics also cut down on development time and cost, allowing the developers to focus on other aspects of the game.