Either way, a zoo will involve a lot of walking around so you should wear comfortable shoes. Remember that the weather might not stay the way it is when you leave the house so taking layers or a rain coat is a good idea.
If your mum is used to carrying things (money, spare make-up, her wand) in her robe pockets she may find that the pockets in muggle womens' clothing aren't spacious enough. I can lend her a handbag, if you want.