Yea, Mrs Brown you've got a lovely daughter, which you would say in the 60s or even 70s when you were courting a girl and would also woo over her mother at the same time. Winning over mom score vital points in case dad think you are not good enough so the strategy then is to get the ladies on your side. Did you ever try this trick? I didn't have to since dad-in-law wasn't the authority of the day.
Today though I doubt many young man would attempt the same trick. Girls are more liberal and independent and mothers tend not to interfere in their choices. However a small number still believe in respect for the elders and curry backdoor favors. Personally I consider it a good PR exercise to raise your stake in future mom-in-law's good book. Praising the daughter's good look suggest the mother is beautiful too, and of good stock. It also recognize mom as great character builder. Personally, I encourage all young men out to court any girl to first court her mother. And to make her treat you like her own son. That way you will be assured mom-in-law will always take you to heart in any marital discords later. As a father with 2 married daughters I say the respect given by my sons-in-law earn them my love and respect too. It is a 2-way street. I'm sure my missus sees it the same way. After spending so many years to raise them and then lose them to other young men it is natural to expect assurance that their wives will be well cared for, and that we will not be neglected.
Now that my daughters are also parents of daughters as well maybe they will think of the days years from now when a young handsome boy will say to them, Mrs. Xx you've got a lovely daughter, just as lovely as you ... Love and respect should never go out of style. Maybe my missus will love to have this said to her one day, Mrs. Yew you've got a lovely grand-daughter ..... and she'd be beaming ear to ear for she indeed spent much time grooming them in their childhood years. And the good breeding would have come in great part from her.