We've had a random Chinese manufacturer claiming that they're working on Switch Mini accessories based on secret information, which didn't exactly have people feeling like the info was legit. Now we have something a bit more trustworthy, as it's supposedly from well-known UK accessories publisher, Bigben. As you can see above, they appear to have a silicon sleeve in the works, and the box itself says Mini Switch on it. As long as this item is legit itself, you can't get anymore clear-cut than that!
There was also a render leaked as part of this Bigben project, which shows how the sleeve would look on the Switch Mini. We'll try to get comment from Bigben on this situation to see if they confirm the leak.