There is no formal dress code for Toledo Cathedral but my general advice is that you should dress appropriately and not take a chance to be turned away. After all you will have gone to Toledo for a day trip most likely, and you do not want to be turned away from the Cathedral which is one of the main sights of the city.
Even in the summer, women visiting Toledo Cathedral need to make sure they cover their knees. This can be tough to follow with all the climbing involved in the hilly streets of Toledo. But you must carry something that will cover bare knees and shoulders.
Make sure to wear half sleeved or full sleeved shirts or T shirts. Sleeveless tops are considered inappropriate even for men in Toledo Cathedral. Also take off any head gear before you enter.
More strict dress code on Sunday
Toledo is a traditional city and the Cathedral is very strict on maintaining dress code during mass. So if you are visiting for mass you will have to be extra careful about what you wear, compared to other days of the week. Unless you are religious, my suggestion is to anyway avoid visiting Toledo Cathedral during Sunday morning mass; you will not be able to move around and see the magnificent interior. Sunday tourist hours begin at 2pm.
Finally one practical advice, wear good shoes as there is lot of walking to done at Toledo.