Many columns in the Idea list view can be sorted, however, the Categories column cannot currently be sorted. One workaround would be to use the Idea category filter to help narrow in on areas of interest.
Given the workaround noted above, along with high level of complexity to sort this specific data object, we are unlikely to make updates here. We will mark as Unlikely to implement to keep this open in case we learn of a compelling use case where the time investment would be worthwhile.
Can you add the ability to filter to just ideas that have no category? Seems like a good compromise with relatively easy implementation.