Wow, this is beautiful <3
I'd say this isn't a dating sim yet, but it's better than a dating sim! I would love a game that spent less time on(or even skipped over) the "get a date" stage, and more time on the "been dating a while" stage :) Maybe a "what level do you want to start on" kinda option?
The demo has very limited content, and the usual supply of demo bugs(May often stops reacting and just keeps playing her base idle animation, one time watching the first Godzilla movie blocked watching the second, and try talking to her with the speech bubble in her room then over the next few days try watching movies), but it's already at the "where did my hour go" level :)
On the subject of dating sim mechanics, the big one is covered by the TVTrope RelationshipValues. (Really, you can learn a lot about any media genre by spending a saturday afternoon looking it up on TVTropes.) One way to think of it is: How much does she think you're a goofball? How much does she like goofballs? How much does she think you're a gentleman? How much does she like gentlemen? Etc.
A trick with idle animations is to make sets of 3(or more) sub-animations that can all tween into each other, then after each sub-animation pick a random one to go next. 3 idle sub-animations like this will look better than one idle animation 3× as long.
Any plans on adding chargen, at least a gender choice? As a lesbian, every time she referred to me as "guy", and every time the mouse dude appeared in the shot and I had to remember that's supposed to be me, it was jarring.