It's now normal to shower/bathe and wash, etc, daily - even in the 1980s, a lot of people had parents who believed one bath a week was all that was needed, most didn't even have a shower, things such as you shouldn't wash your hair during a period were still commonly said and the only skincare products were monstrously stingy chemicals that would practically burn the skin around any spots.
Pretty much every teenager had greasy hair and a lot of spots. And they stunk.
When I was a teenager, there were a few of my friends who only had a couple of spots throughout their teens - even then, I could see that they had a more comfortable lifestyle, complete with daily showers/hair washing and always had clean, ironed clothes.
In the years since that I've been working in secondary education, I've noticed that teenagers take far more care of their hygiene - and they rarely get more than a couple of smallish spots around the age of 14, just as my friends did. And the few kids who don't quite grasp the importance (or their parents still don't believe in it) and smell bad are almost wholly the ones who also have greasy, dirty hair and a large number of spots and boils.
I believe that the change in expectations and facilities at home led to this massive reduction in acne, (as it's certainly not due to any improvement in diet or exercise or changes in average hormone levels).
They just don't leave staphylococcal bacteria on their skin for long enough for it to find its way into hair follicles. They aren't coated in excessive sebum or sweat for long enough for bacteria to cause odour.
There will always be some who are unlucky and have an entirely separate medical condition (cystic acne) - but the majority of youngsters are not getting the same level of skin problems because their hygiene habits have changed - and by starting those habits slightly younger, this means that they don't experience the issues a lot of us did.
Your DD's dad doesn't want her to be the 12 year old at school that smells bad, the 13 year old covered in avoidable boils and the 17 year old plastering on foundation to try and hide scarring. Explain this to him - that the stuff about 'spots are nothing to do with cleanliness' isn't true - that whilst she might get some, they will be fewer in number and severity, as long as she continues doing the things that she is enjoying now.
She needs to keep clean. It's not sexualising her to allow her to enjoy keeping clean. She's not doing it to attract boys, it's hygiene.
(so he can bugger off with complaining she is growing up too fast)