It's been awhile since I read the Dressrossa Arc, so it might be in there (but I skimmed it and didn't see this specifically mentioned), but I was wondering: does Sugar turning someone into a toy make that person unable to use their devil fruit? Not to, like, hurt someone (since she basically uses the 3 Laws of Robotics) but in general/for day-to-day things?
It's hard to say. Hypothetically, a toy could still hold a devil fruit power since Dr. Vegapunk made it possible to give items devil fruit powers. However, the only Devil Fruit holder turned into a toy was Robin. Robin is seen very little as a toy: just right after she becomes one and the following episode (episodes are manga, but she is seen just as much in the anime). If Robin could use her power, her arms would likely be weak toy arms, so she never even tries to use it. This ultimately means we can't yet know the answer for certain and as far as I know, nobody has asked Oda. Both answers are equally likely.
I may be wrong on this but I don't think we have ever seen her powers used on someone with a devil fruit and as a result we never see any manifestation of devil fruit power nullification. Because of this I would have to say her powers do not nullify devil fruits as there are no mentions of it in the anime manga or wiki. The link below is to the One Piece wiki where you can read about her abilities. http://onepiece.wikia.com/wiki/Sugar
As per the series and most of the cases, people can't undo the effects of devil fruits no matter how small or great they are, unless they overcome it using Haki, as seen in Law and Blackbeard's battle when Doc Q turns Law into a woman using some disease with his Sick Sick Fruit.
There's still more to Sugar's power though, such that she can make contracts with people who she turns to toys which also limits the toy's abilities.
There are more users in anime whose devil fruit abilities can affect living creatures, such as
- Wapol's Munch Munch fruit, which allows him to combine something he eats with his body or entirely something new as a part of his body, like cannons and guns, as shown in anime, even living creatures to fuse them.
- Tsuru (one of the Navy's legendary trio) has Wash Wash fruit, which transforms even living creatures into washed laundry.
- Inazuma's (Revolutionary) Snip Snip fruit allows him to transform anything he touches into a paper-like to snip through it easily, its effect is still not shown on living creatures, but it's highly likely that it can affect them as well.
Being turned into a toy prevents a Devil Fruit user from using their powers. This can be seen in the case of Robin. Sugar is able to transform someone into a toy by touching them. In episode 673, she turns Robin into a toy by touching one of Robin's sprouted arms. These arms still count as part of Robin's body, so Robin turns into a toy, and all of her arms holding back Sugar instantly disappear. This indicates Robin's power was negated by being turned into a toy.
In episode 674, after Robin is turned into a toy, she says,
I've been turned into a toy. [...] There's nothing I can do anymore.
If Robin could still use her power in some capacity, she would still be able to help. What Robin says indicates that she cannot use her power.
In lieu of any evidence to the contrary, we can generalize Robin's predicament to all Devil Fruit users.
We can speculate on why that is the case. A Devil Fruit typically transforms a person into something (a rubber man, a spring man, fire, sand). Sugar's power also transforms a person into something: a toy. If she is able to touch someone, then she has the upper hand, and she is able to subjugate them to her power. But what would happen if a Logia type, for instance, was still able to use their power? They could transform into their element and would no longer be a toy. It doesn't make a lot of sense to allow this from a narrative perspective.