Quality problems in this png illustration

This png illustration was not accepted because it had quality problems. Can anyone tell me exactly what they are? Please

By @My little childhood

The missing antialiasing on the lines has been discussed. But your cutout ids not creat neither: 

The read shows a transition between transparency and non transparency. You see, you have transparency where you should have none. You nee to address that too.

Most lines are too jagged as @Jill_C mentioned, while others are too soft. 

Zoom in to 100%+ and you'll see that the outlines are all very jagged lines. Those should be smooth. Why is it a PNG file? That file type should be reserved for assets with transparency.