If I had to pick (which, like you said, is extremely hard), I would personally consider Terraria's soundtrack to be electronic orchestral music. When you listen to a lot of the sounds in the music (especially the boss themes), you will start to notice a lot of crossovers between the two genres. I wouldn't say chiptune however, because the way chiptunes are made is through very specific techniques, usually faithful to the console or PC of orgin e.g, NES, famicom, sega genesis, etc. The music found within Terraria most definitely could not be produced with the limitations of chiptunes, therefore eliminating chiptune from your options. Ambient could certainly be used to describe many the themes such as Overworld Night, Eerie, and Ocean, but on a lot of the more hectic themes like, Underground Crimson, Lunar Towers, and Boss 2, it would be out of the question to consider them "ambient." I would stick to something along the lines of electronic orchestral music. You should go to Terraria Soundtrack | Re-Logic and listen to the various different themes that I listed to decide for yourself whether you agree with my opinion or not (A lot of the themes start slow, so it might be best to skip around within each song.)
I hope this helped.