I'm a regular. Ballet is less dressy than opera, and matinees less dressy than evenings, generally. But you really can wear what you want. See it as a chance to have fun with clothes, make up, jewellery etc. if that will make the experience more special.
There'll be a big range of clothing and levels of formality. I'd usually wear trousers and a nice top, plus trainers or ankle boots, with a necklace and a smart mask, for a matinee. Or maybe a nice, fairly understated day dress. Some people come in very casual outfits, just normal daywear, and others, especially one-off visitors, will glam up a bit. Recently at a ballet matinee in the morning I saw a woman in a satin full-length low-cut gown and diamonds. That did stand out as surprising, and is way removed from the norm, but she was probably enjoying wearing it, and though people might have blinked at first sight, I don't suppose anyone cared.
The cloakrooms are still closed so take a jacket or coat you can keep on your knees comfortably. Avoid clothing and jewellery that makes a noise or rustles if you fidget (that rules out my pleather trousers), and if you're in cheaper bench seats it's probably best not to wear a large skirt; there won't be room.
Have a wonderful time!