I don't think anyone bases a long term relationship on looks alone. Looks attracts people to you, but it's your other qualities and also what YOU'RE looking for with a spouse that begin to become more important than physical beauty. I mean, I kind of feel like we have the same opinion on the matter, but I guess you also being a mom gives you a different perspective. I'd be floored if my kid said that to me, because it's not a matter of looks but a matter of perception as people are going to think your teen isn't washing their face or is eating poorly or doing something generally unsanitary.Attraction is different and yeah--I do think it's important for a relationship.
But some of the happiest couples I can think of aren't necessarily going to win any beauty pageants, and besides--looks fade. I guess I'm thinking about long-term relationships like marriage. And while looks aren't harmful, perhaps those couples had more in common than looks which is why they seem to have had a stronger foundation that is more lasting. Beauty is subjective though and attraction is also very individual, but I am referring to what is considered conventionally beautiful/handsome.
I just don't think it helps much to worry about how you look for others, as beauty is subjective--but how happy you are with your own appearance since that will affect your confidence and your overall happiness. Looks can be important, but for most people they are not as important as young people sometimes are made to feel--either from the cosmetic companies or just watching fellow young people mess around. And even if you manage to impress someone with your looks, it's no guarantee that they are the right person for you (probably very little likelihood since looks are so superficial).
I don't think people should seek to be in relationships with people who they aren't attracted to though. That is not what I'm saying. I've also had a hard time arguing with my teenager about using acne soap because they assert "looks shouldn't matter!" So it's not really simple, I don't think.
But yes, a good looking couple may not necessarily be the happiest couple. Personally, I do think looks are initially important, but then other factors like compatibility (social, emotional, sexual, cultural, etc), being able to live together, not arguing, agreement on kids (and starting a family), personal values, etc start to become deciding factors in whether you'd want to be in a long term relationship with a person. Also, studies on long term arranged marriages/relationships show that happiness comes with time and understanding. You don't even necessarily have to like each other much at first, but simply sharing space with someone successfully and the both of you being able to understand where you're each coming from will lead to a happy relationship.
So yeah, whether looks are important or not can be complicated, but the secret to a happy relationship is actually very simple. Time and understanding, that's all it takes. Well, and you have to find your spouse to be attractive as well, but that's my additional condition.