We can't tell from the photo what caused the brown tips, but yes, they can be trimmed off. Physical damage from a snapped twig, minor under- or over-watering stress, or an inconsequential infection or pest activity could have caused the browning. Treatment is not needed at this point and the rest of the foliage appears healthy, but if this spreads more extensively later, feel free to send us updated photos. A more widespread and sudden color change of the canopy to a yellower green or brown would suggest root death, which tends to be related to root moisture levels when it occurs with Arborvitae.

Miri