Nerves of (braided or symmetric) monoidal categories

If you want to capture the structure of the category together with its monoidal structure, you may need a $k$-fold simplicial set for $k>1$, i.e., a functor from $(\Delta^{op})^k$ to sets. One of the simplicial coordinates encodes the composition law in the category, another encodes the monoidal structure, and the rest decribe compatibility between monoidal structures (if the monoidal structure is braided or symmetric). See also Double nerve. You may want to look up work by Baez and Dolan on their periodic table that expresses monoidal categories of various types as higher categories with connectedness properties. In particular, there is an equivalence between monoidal categories and 2-categories with one object, and an equivalence between braided monoidal categories and 3-categories with one object and one 1-morphism.

For plain old monoidal categories, you could regard them as a bicategory with a single object and use the Duskin nerve. For braided or symmetric categories there might be higher nerves you can take, but I'm not sure how well those work. You might be better off using the k-fold simplicial sets that Scott suggested.