Most essential oils are fine to use neat. I would always, however, advocate for diluting oils when using them on babies or pets. There are some oils that are considered hot – such as Cinnamon, Oregano, Cassia, Wintergreen – and they should be diluted every time you use them.
A small percentage of the population may also have a sensitivity to Peppermint essential oil. If this is the case for you, by all means dilute!