After liposuction Do i have to wear the compression garment directly on skin or should i use it on top of my undershirt?

Anything that is worn against the skin and under the compression garment should be very form fitting. You don't want something bunching or wrinkling under the garment. I would ask your surgeon about this and show him/her how it fits. Good luck!

You can use it
either way. I would recommend using it in the manner that is most comfortable
for you and what your surgeon recommends.

Compression is always encouraged after liposuction to reduce swelling, prevent bruising and fluid collections. Compression is compression so I don't believe it matters what you have on under the garment as long as it fits correctly.