All Mini-bosses is a part of the Full Clear category, and can also be a part of all flags and all bosses, so there's no point in adding that into the regular categories. Plus, if it were added, it would be defined as a category extension.

And, all minibosses is a part of the category extensions under the king dice level.

I mean, you could still just speedrun Mr. Wheezy regardless of the fact that there's no IL for him. Not being able to submit a run for Mr. Wheezy doesn't mean much anyway if that's what you're speedrunning. Having ILs for every single mini-boss would be convoluted and unnecessary.