Haha! Congratulations on making a new friend for your mom. Sounds like just the excuse she needed to actually get to know your neighbour.
In this case not really a lack of filter on what is said (which kids do also lack) but a lack of social understanding on when and with whom we even start a conversation at all.
As adults we have all these unspoken rules before we decide to talk to someone. Do I know them? Do I have business with them? Does it look as if they want conversation or not? Am I going to be inconveniencing them?
Kids on the other hand will jump right in without a second thought, and that's kinda great. We should probably all be a bit more kidlike sometimes!