You can determine whether using an AI humanizer is considered cheating by reviewing your school's policy and considering how you are using the tool. If your school prohibits AI-generated material and you use a humanizer to make that material appear to be your own work or to disguise its origin, you have violated academic integrity. However, if you use it solely to improve readability, it may be acceptable.
The main issue is authorship. Ask yourself whether the core ideas, arguments, and overall structure of the paper were created by you. A humanizer is a type of paraphrasing tool because it refines the language used in your paper, but it cannot replace your original thought. If you'd like more information regarding ethical use of AI tools I'd recommend looking further into it here, read Top 10 Prompts to Humanize AI Generated Text. Refine your writing while keeping your original voice and meaning intact using Quetext's AI Humanizer.