Your Mum has some very outdated ideas. I wonder if her own mother filled her head with this nonsense, and now because she followed the 'rules' she expects you to do the same. It has been decades since women in their 30s and 40s were 'expected' to dress frumpy. The world has moved on!
^This. I think your mum's mum, was probably of the era when women reached mid 20's and/or became a mother and lopped off their long hair and were expected to dress completely different to the "young adults". So your mum followed the way your grandmother did things and is now trying to impose that on you, but hasn't realised things have moved on.
You certainly can have your arms out, wear mini skirts, show masses of cleavage, whatever you want now. I'd say the only things people tend not to wear (but some still do) past their mid-late 20's is slogan t-shirts, tops that show your bra straps, mini skirts so short they are practically a belt. Other than that anything goes for pretty much any age.
Just relax and wear whatever you want/ what flatters you and makes you feel good. Don't put age limits on clothes.
Do you have a friend, partner, sibling, etc who would be honest about how things look on you but doesn't follow your mother's views? That should help you find your style again, after your weight loss.