I’ve been to 3 graduations in the last 5 years. Most mums were in dresses like the ones you linked too, with a smart coat or jacket if it was cool. Some were in smart trousers and a blouse.
Only a few wore MOB type outfits , I didn’t spot any fascinators.
Most dads ( and male graduates ) were in suits / collar and ties or national dress . A few brothers were in jeans with a casual open neck shirt / jacket ( so not scruffy or casual ).
Some brothers wore 3 piece suits , as they are in fashion.
I didn’t see a single person in jeans / tee shirt / trainers ( although I might have missed a small child ).
Women graduates were more dressed up with heels, lots of make up and hair “done “, as is the current fashion for women in their 20s and 30s.