As the only living trees known present in the Capital Wasteland are in Oasis it is possible that they were grown here, though it is just as likely that it was created through genetic experimentation. Rivet City scientists are capable of growing fresh, radiation-free fruits and vegetables, so it's clear that fruit-growing climates can be replicated with the proper technology. There is no in-game description of the actual origin of the mutfruit.
Unlike crunchy mutfruit, mutfruit is easily recognizable by it luminous vivid color, even in dark conditions. With blue splotches at the end of the fruit and purple going towards the stalk, you can't miss it.