Hi!
Littlefoot also mentions his mother in the 5th movie's song "Always there", and it is certainly interesting to take a look at what the song says, and that it makes Littlefoot think of his mother rather than his grandparents.
In general I don't think that Littlefoot is frequently depressed thinking about the "might have beens" if his mother had survived.
All this is just interpretation on my part which can't be proved, but I think that Littlefoot does have sort of "melancholic" moments at times during which he thinks of his mother, or maybe even senses her presence (listens to the voice of his heart) as he did in the original movie.
Yet I don't think that Littlefoot permits these moments of melancholy to take over his life. Even though he is often more earnest than the others are usually, he is certainly not a totally depressed character.
