She won't retain this ability for long
A human body is constantly renewed, that is, atoms enter and exit the body, cells get replaced, water lost and replenished, etc. And whatever atoms that would enter that witch's bosy will still retain normal mass. Therefore, after she'd eat (which would require her to carefully calibrate her movements, because there will be less inertial mass to move when moving a spoon from the plate to the mouth), her body will start assimilating normally weighing atoms into itself, raising its inertial mass to what was before the accident. The same applies to breathing (actually this will start affecting her right away!) and drinking. The lightweight atoms will eventually be ejected from her body by normal means.
A human requires about 2 liters of water per day to maintain his water balance, also a human requires to breathe in about 11000 liters of air per day, but only about 5% of incoming oxygen is absorbed in the lungs, thus the amount of oxygen transferred into the body per day is about
11000*0.2*0.05*0.001428 = 0.15708 kg. Let's count for the witch to eat 2 kg of food per day, including water, and assume that 90% of eaten atoms actually move through intestinal barrier into the body. Thus, 1.957 kg of the body's atoms get absorbed into the witch's body each day, and in case of her not gaining body volume ("weight" for a normal human), about the same mass's worth of atoms leave her body during the day by various processes. Those atoms that leave the body will be distributed about proportionately between normal and lightweight portions of Clarabelle's body, e.g. if her body is composed of 25% normal atoms and 75% lightweight atoms at the start of a day, at the end the composition of all the atoms exited from it will contain 25% normal and 75% lightweight atoms, give or take a few. Therefore, if right after the incident Clarabelle weighed 4 lb, and continued to eat normally, after the very first day she will weigh
(1.6+1.957-(1.957*1.6/53)) = 3.498 kg or about 8 3/4 lb. The next day she will lose less than 100% of lightweight atoms, thus will gain less mass than 1.9 kg, yet she will still gain mass even if not gaining weight. This process will continue at an exponential rate of e^(-x), making her weigh half the weight before the accident after 14-15 days, three quarters after a month, and after three months she would be almost indistinctive from before all this happened.
In the meantime, however... she will suffer a set of inconveniences
First, her inertial mass will make her movements too fast, making her steps too light and throwing her lightweight body towards the ceiling with each step. If she'd just stay within her house, it won't affect her for long enough to harm her health. Yet, if she'd go outside, she can end up tossed by the harsh wind, whacked and toppled over by some thrown stones or other incoming physics, and should she somehow end up in water, everyone seeing her float like a balloon would yell "WITCH!!!". However, this could be used by her to an advantage, as she will be able to climb trees up to the very top and actually run on treetops like a squirrel, thus avoiding capture and bonfire sentence. This stage won't last long if she'd eat well, about 2-3 days and the wind would stop being a threat, for example.
Second, she will have to adjust her almost-weightless body (while it is so) while holding and operating stuff like pots and buckets. For example, a 5-liter pot filled with water will be heavier than her entire body, and while pulling it out of an oven or furnace along horizontal surface won't be a difficult task, because she won't instantly get weaker, and after all she still can use her legs against that oven instead of the floor to leverage her strength, lifting it upwards will make her unstable, at least until she couldn't get her feet under the pot, or rather, under the mass center of her and the pot, which will be a little more than halfway towards the pot. I say this would be minor one, as her strength will no longer be spent on lifting her arms together with the pot, so instead of 15 kg, comprising of 10kg of arms and 5kg of pot, she will have to lift only 6, yet developing a proper pulling technique would take a while.
Third, some interesting physics would interfere with her biology while atoms replenishment will be underway. For example, if blood water will be a tenth normal and 90% lightweight, the normal tenth will tend to sink, inducing countercurrents in veins below heart level that can potentially weaken the witch's body. Thankfully, this effect even at its hardest won't inhibit blood flow (or lymph flow) at any rate of normal/lightweight atoms distribution, because lower weight of altered blood will actually help blood system to maintain blood flow. Yet, the flotation effect could invoke various problems on small scale (intracellular effects) which I am unaware of. Overall I assume that the following month Clarabelle would experience weakness, sweating, lack of appetite and possibly fever or hemorrhaging due to increased strain on immunity system which will react on some cells performing badly. This likely won't lead to her death, though.
Maybe there will be more effects, but they will likely cease after about three months, should Clara survive.