They're supposedly some form of magical super food, right? Not like some all-cure medicine, as they can't cure ailments nor healed over wounds. It's like they super charge the body's natural healing or something. But, given that they also state to be able to sustain someone for a weak, if we assume the magic isn't just to sustain a body, but to magically produce all the needed nutrients for it to sustain itself for so long somehow, then could a Senzu Bean cure malnutrition and restore the body to what it would be like normally?
The Senzu Beans are mainly used to replenish Physical Health and Energy instantly.
- However, during the android Saga, the senzu bean was capable of healing him despite Dr. Gero literally ripping his torso apart.
- Also, in future Trunks Timeline, it was implied that Future Gohan wouldn't have lost his hand if he were to have eaten the Senzu Bean himself instead of giving it to Trunks.
- Even in the latest Tournament of Power arc, Vegeta had a black eye after his fight with Jiren which was instantly cured on taking a Senzu Bean after getting eliminated.
Hence, with regard to the first part of your question,
- It is not an "All- Cure medicine". It was stated in the Android Saga that Goku had died on account of having a heart virus and that there was no cure available at that point in time. So that implies that the Senzu Bean is definitely not a cure-everything drug.
With regard to the second part of your question,
- I believe the Senzu Beans wouldn't necessarily cure malnutrition or per-say make a person Healthy. It would only reset the person back to the exact state he was at before receiving any injuries.
- This answer is very hard to determine considering the fact that these Senzu beans are mainly consumed by Fighters who all are Physically Fit and ideally follow good Nutrition. At the same time, the rules of Malnutrition applied to Humans would be significantly different compared to other races like the Saiyans(Since they produce a massive amount of energy and in turn have a massive appetite, and we see Goku and Vegeta pig out on food and are yet relatively healthy).
- Hence the one person I would like to consider under the mentioned circumstances is Yajirobe. Yajirobe is definitely a lot unhealthier in comparison to the rest of the Z-Fights and has excess body fat. So ideally, if a Senzu bean is capable of restoring a Mall Nourished Body to it's normal state, it should be able to restore Yajirobe to a more healthier state which clearly doesn't happen.
Then again, the probability of finding an individual in the Dragon Ball Universe suffering with Malnutrition is pretty low considering the obvious theme/setting of the show. So one cannot state a definite answer with regard to the same.