If you are having spikes, I recommend smoothing them with a smooth brush by clicking at the top of the spike continually first and then applying this method to prevent them from generating again on changing shape keys. (Please check if they generate again on changing shape keys before applying the method and if not, you can still watch the video to learn about a new brush)
The Problem Video
https://drive.google.com/file/d/1heHcEr5EcAlw1Of17QnLz87-BDyri6MQ/view?usp=sharing
The Solution
https://drive.google.com/file/d/155jmCP1bq4caK_PlJT5r2b6Btf3zm6oc/view
