More or less, yes. Tropes are tropes for good reason.
There are counter-examples, though, even in straight Disney stuff, for example Tangled which kind of subverts the trope in that the main character believes she only has a single parent when in fact her real parents are still searching for her. (Of course, "Flynn" comes from an orphanage, so the trope sneaks back in anyway...)
I wouldn't be surprised if a lot of it is due to the compressed nature of storytelling film in the first place. By limiting the number of familial relationships, the ones you choose to keep become that much more important and you have fewer distracting side-elements, which can be vital when you're talking only 80-90 minutes of run time for an animated film.
Walt Disney suffered a horrible tragedy when a badly installed hot water heater caused carbon monoxide poisoning to his mother in a house Walt has bought for her to live in.
It is a popular belief as to why you see so many stories with deceased mothers specifically or parents generally.