It sounds like you might have three separate concerns, each of which might be addressed differently: abdominal girth, core strength, and jiggliness.
If you are concerned about abdominal girth, your problem may be visceral fat. While people say you can't spot reduce, visceral fat (located around your organs) is different from subcutaneous fat. It is more metabolically active and responds differently to diet and exercise. Rather than trying to do stomach exercises, visceral fat may respond better to blood sugar controlling diets (lower carb/paleo/mediterranian) and cardio, especially HIIT. I'd suggest reading about healthy waist-to-hip ratios and researching ways to reduce abdominal fat. In my case, eating lower carb and exercising more seemed to improve my waist-to-hip ratio.
If you are concerned about core strength, then by all means do core strengthening exercises. Core strength is important for balance, stamina, life in general. Proper crunches, and push ups if you can do them (or take steps towards being able to do them) are ways to increase core strength.
If you are concerned about jiggliness, this is probably a side effect of weight loss. You may have loose skin. Over time, it will tighten some. There is a sticky thread devoted to the issue of loose skin if you are interested. The jiggliness could also be due to a phenomenon that many 3FC'ers have noticed. Many of them swear that their fat gets jigglier just before they shed pounds or inches. Almost like the fat is being replaced with fluid then the fluid is drained.
Here's one of the threads about it: