Is "投个好胎" a proper set phrase/idiom? What does it mean?
投 (v); plunge into

胎 (n); pregnancy

"投胎" (plunge into pregnancy) is a compound word. It means "to reincarnate"

Olden time Chinese believed after people died and became ghosts, some of them were allowed to reincarnate and reborn into the living world 

"投个胎" break up the compound word "投胎" and treating 投 as a verb; 胎 as a noun. When 胎 is a noun, we can add classifier '个' (a)  before it to indicate indefinite 

Similar phrase: 投幾次胎 (to reincarnate a few times)

different from "投个胎" (to randomly reborn), we can have “投个好胎", which mean "to reborn into a good family"