Frozen was a rushed clusterfuck because of the constant rewrites and plot changes. For example they literally COMPOSED "For the First Time in Forever" in June, when the premiere was already set to be in November. That's a crazy tight schedule, but they chose a plot that "clicked" despite being rough and unfinished and full of holes and resulting in a rushed animation they had to deliver half-baked because there wasn't time to do it properly over having enough time to hone a plot that felt unsatisfactory.
As a result, the writing is kinda clunky in places, and the animation looks really shit in some scenes. It looks fine in other scenes - probably the ones that made the cut from a previous story version and had had enough time to be done nicely, but in many scenes, surfaces, textures, shadows, crowds etc the animation quality is visibly sub-par for Disney. People tend to say the movie "aged badly" but it was just a shit rush job even for its time.
Essentially they ended up spending as much time animating the 7-minute Frozen Fever short as they did the Frozen movie proper. It's insane.
Apparently a lot of the people making it kinda expected the movie to tank, which is also why Disney was so wholly unprepared for the massive demand for Frozen merchandise around Christmas 2013 when everything Frozen was constantly out of stock and they just couldn't pump out dolls and dresses fast enough.