Adapting this game would be challenging since the story seems too long and episodic for movie form. Disney ran into that problem while trying to adapt The Snow Queen and it took them 75 years to make a proper movie version of that in the form of Frozen. I think Studio Ghibli would find making a two hour movie out of Undertale tricky, but not impossible. It would have to elaborate on Frisk's backstory, give grander instrumentation to the soundtrack, and give the action scenes better visuals than the game ever could. As long as this version of Undertale does a significant amount of things different from the game (but not to the point where its unrecognizable), it should be fine. You don't want this movie to be redundant.