No you should not ask permission to change a toddlers diaper.
When a toddler needs a diaper change then the diaper needs changed regardless if they want to give permission or not.
A toddler really does not understand what permission means anyway.
If you ask most toddlers if it's okay to change their diaper they will flat out tell you no and run away.
You should let the toddler know you're gonna change their diaper soon though and that there's no option.
Sometimes you have to fight with the toddler to get their diaper changed or get them clothed or bathed etc but it must be done.
Asking a toddler or child permission to change their diaper just makes it seem like the toddler or child is the boss.
Never ask a child or toddler permission to do something they need to do.