I mean rather depends on how you define useless. Touko may not give a rat's ass about or be any good at housekeeping (there being a rather obvious causal relation between the two) but near as we can tell she's got a nice career going and seems to be pretty happy with her current lifestyle.
That's more than a lot of people can say.
Living in utter squalor as an adult is a failure to be a functional adult and a detriment to your health and safety and in this case the health and safety of the minor she agreed to be a caretaker of who instead is forced to play caretaker to her.
She's a white-collar worker who employs a maid service to clean up for her, as many privileged career professionals who don't have time for housekeeping do. That's not a "failure to be a functional adult".
She just needed to have her maid come by more than once every two weeks, clearly...