Yes Ramen should replace tobacco as the most popular prison currency since people are not allowed to smoke in prison the way that they once were. Because people are not allowed to smoke in prison anymore, they need to find another prison currency with which they can barter with other prisoners.
Ramen is very popular among prison inmates because the food served in jails and prisons is not very good and portions are limited. The Ramen noodles can be consumed cooked or dry and are very filling. These noodles may not be the healthiest choice for people who have other options, but inmates have few food options.
In prisons, people should have the right to smoke. It doesn't really matter what currency people use. They will use whatever is in short supply and that they want. Prison runs on a barter sytem because inmates are not allowed to transfer money to each other, and are often not allowed to trade either, anyway.
Ramen should not replace tobacco as the most popular prison currency. Ramen is barely nutritious, lacking most of the essential vitamins and minerals as well as other nutrients the human body needs. It is mostly empty carbohydrates. Though tobacco is not a form of nutrition, it is craved by some and can calm the nerves of some people. People that are craving nicotine because they are used to smoking cigarettes at home can become irritated without their tobacco use. I think tobacco would make more prisoners satisfied than ramen.