This is quoted from a recap & review of Episode 5 of Walking Dead. I sense that there's an overlap in meaning between "tacked on" and "afterthought". Both of them , in my opinion, connote poor planning or little thought given to the opening. If it's correct, I am wondering if "The flashback comes across as an afterthought" could also work in this case.The cold opening flashback seems a little tacked on. Personally, and maybe I am in the minority here, I enjoyed not knowing about what lead up to the zombie infested world we see each week.
Tacked on: to add something that you had not planned to add, often without much preparation or thought
Afterthought: an idea or plan that was not originally intended