As for the person who would be disappointed if 'even' the woman down the road 'just' gave them soup for lunch - how entitled are you?
Ahem, that was me. I am not entitled at all (at least I don't think so). I make the effort to make a proper meal when guests visit. It doesn't mean I spend hours slaving in the kitchen (although sometimes I do). But it does mean giving it proper thought, having a starter, main and dessert which go together and providing good wine.
It is sending the message that 'I care enough about you enough to make an effort'. It seems to be the norm, at least with my friends, neighbours and family to host in a similar way.
When the neighbour down the road invited me for lunch last time, she served some market bought puff-pastry type starts, a main of lamb for the meat eaters and a zucchini bake for me, lovely roquefort cheese and I can't remember if they was a dessert. Plus good wine and a nicely laid table outside.
It probably took under an hour to do all the prep, but the impression it left was lovely. What if she had served just soup? It would have been perfectly fine if it was a spur-of-the-moment last minute invite. Otherwise, it would have been just strange, given the norm around where I live.
I guess it depends on what OP's mum prepares for lunch when she has OP over. If she makes a proper meal, then OP should have reciprocated accordingly or if she had no time, just taken her parents out for lunch.