'Excelling' or being 'great at' is being scouted for the club or playing for the county (or country like Zara).
'Excelling' at art is putting out pieces for auction like Charles and Zara.
A prep level piano skills as you would have it from the school kids at the town hall is not 'excelling' at anything either.
At Marlborough sport is compulsory around 2 afternoons per weeks. Then there is the weekend matches. For Shell/Remove extra curricular like music etc. is also compulsory.
By your logic everyone at that and most boarding schools 'excels'.
Concentrating on only 3 subjects with a tutor (possibly above the regular tutor at hall/toys/study time will surely make anyone pass, surprised it's not all A*.
As for KM 'concentrating on her childcare'. She has an army of nannies (they even have their own house, right next to her 'cottage'). Plus you could tell from her pancake day, she has never held a spatula or pan in her life, despite the PR nonsense that she is such a brilliant 'homemaker'.
Yes, it was reported plenty of times that HMQE was strict about weighing down hems.
She has never had a proper job except working for her mum and her mum's friend in internship.
Stop trying to big up a mediocre person.