So time to time I face this problem when creating twinkling star: I draw an ellipse, then go to "Effect" > "Distort & Transform" > "Pucker & Bloat" > Done. It looks sharp, but once I zoom in you can see that the ends are weird and square. Can someone help me to fix this problem, please?
Do not use that complex tool for a job which should succeed by using something much simpler. Use only the pen and the direct selection tool (the node tool for those who prefer to use Inkscape).
Draw a 2 node arc with the pen. Hold Shift when you drag the handles (Ctrl in Inkscape) to get perfectly horizontal and vertical handles. Make horizontally and vertically flipped copies of the arc. In the left the (temporarily) red curve is drawn, the 3 others are flipped copies.
In the middle the pieces are dragged so that they snap together and joined to one closed path. Joining means in Illustrator and Inkscape selecting all and performing join together; that's Ctrl+J in Illustrator. In Inkscape one must select all nodes with the node tool (=drag over) and click button "Join at selected nodes" once.
Too symmetric version is not good in all cases. Especially in cartoonish images one may want several different stars which have different sizes and slightly different geometric forms. In illustrator and Inkscape it's easy to drag the cusp nodes a little, adjust the node handles or drag directly the curve to make quickly different versions. In the right there's 2 variations.
If you are a beginner this is a great exercise to develop your curve editing skills.