I don't see that line as rude at all. Remember, during this time, if memory serves me right, Littlefoot was lonely. I always thought it was normal for some only-childs to wish they had a sibling to keep them company, or to encourage them, or whatever. I don't see this as a sign of rudeness or ungratefulness. I think Littlefoot had every right to say that line because, as noted, he was just lonely. He does not appreciate what his mom did any less, and he wasn't being rude.