There is no strict dress code for visiting Fushimi Inari Shrine, although the shrine specifically prohibits walking around the grounds in wedding dresses or other costumes that are inappropriate for worship.
Since this is a hike up a mountain, I recommend wearing modest and comfortable clothing. Wear comfortable shoes suitable for walking or hiking, as Fushimi Inari features a 4-kilometre trail up Mount Inari.
Please always be respectful within the shrine grounds, keeping noise levels low, avoiding littering, and following any specific rules or instructions provided. If at all possible, avoid selfie sticks, don't leave strollers unattended apart from in marked areas, and don't prevent worshippers from passing by taking photographs/filming on narrow paths.