Stopping to chat to people on a walk/ in a shop/ waiting for the bus - fine. Not my thing particularly (I'm 45!) but would be friendly and pleasant if someone wanted a chat. I hate when someone seems to want to spend the whole train journey or whatever chatting, if I'm travelling alone then that's precious time for me to read or just 'be', and if I'm with friends/ family then I'm probably wanting more to catch up with them. However there's those amongst my loved ones who can think of nothing nicer than to spend an entire flight finding out exactly where Peter and Margaret are going to, how many times they've been before, how they got to the airport and where their grandchildren go to school. I'm happy to leave them to it, not embarrassed by it but don't particularly understand the urge.
Probably would have been a bit embarrassed or would roll my eyes if it was my mum when I was a teenager (although my mum was more of the 'why do they always want to talk to me? I must have one of those faces!' types). Not sure why, just an awkwardness of youth thing.
However, my mother in law would have done exactly what you did. We're currently buying a house and they've already done a few drive by missions, driving us mad with their opinions on this that and the other that they've spotted from the roadside. If they'd happen to have seen the owners they absolutely would have got out and had 'a chat' and I would indeed have been annoyed and embarrassed. They've already spoken to one of the neighbours they managed to catch unawares whilst he was out trimming his hedge! (She was round so often before DH bought the house we currently live in, before he met me, that she now counts the older neighbour as one of her 'best friends' and said neighbour, whilst lovely, has also felt free to go visit the new house and offer us her opinions on it, as well as reporting back to my in-laws things she's noticed we're up to, all in the guise of being 'interested' of course!
My MIL would have pulled the old 'I'm just a friendly person who is interested in people, poor me sad face' too as you have above - but it's not about that is it? Chat and be interested to people in queues all you like, but your daughter has been an adult for over 14 years now, this is her house and her decision and conversations with the current owners are hers to manage. Be interested in her, let her tell you about it, let her ask you for your opinion or offer to show you round. Don't be intrusive. If you've spent your life and career being interested in people and what makes them tick as you say, surely you've picked up a bit of sensitivity along the way - it shouldn't be hard to understand that some people would find what you did overstepping, and for you as a mother to know whether your own daughter is one of those people. (I appreciate the one buying the house isn't the one who criticised you - does she know and what does she think? Arguably that matters more than what you, your other daughter or random mumsnetters think of the situation.)