Type-wise, it will show up as a neutral hit (2 for Water x ½ for Ice = 1), which you can tell either by the sound of the hit or the lack of an effectiveness message afterward. However, Thick Fat acts as a multiplier of ½ on the Special Attack stat for the duration of that move, so it will only hit half as hard as a move of a type that truly had no interactions here, like Mist Ball.