From my experience in a few different Communities on different platforms, it is really dependant on your audience. For example, if you are a support community site for a web-based software, you’re more likely to get desktop views. Where if you are more of a community that is focused on connections and not tied to a particular company/product, you might be more likely to have a higher mobile count. All this to say, generalizations on data like this can be hard, as there are many other factors that can go into what device people use to visit a community.
For the community that I manage, we are primarily desktop - 97% over the last 30 days, and 96.5% YTD. We are also tied to a product that is primarily accessed on a desktop as well, so it makes sense.
Imho - if you are looking for more fuel to the fire on this for internal discussions, I would recommend using the data you have today to tell your story, and ensure that any experiences that do get built/created for your Community are responsive and inclusive of all screen sizes.
We did a redesign earlier this year, and mobile was something we were aware of and took into account, but due to our circumstances/data, we primarily focused on the desktop experience.