>>44521539Dual types will always be offensively better than a single type
On the other hand, a dual type may be better or worse defensively depending on the combination.
At the end of the day a single type may or may not be better than a dual type for a specific pokemon, but taking into account the wider variety of properties dual types have to offer for every combination, the most sensible thing would be that monotyped pokemon should represent around 10.5% of all fully evolved pokemon which is the same proportion they represent in all of the possible typings a pokemon can get (18 out of 171). For every fully evolved monotype pokemon we should get 8-9 dual typed ones