At present, it is not possible in OneNote for Mac to hide completed tasks. There has been a Uservoice open for this for nearly three years so even though I would recommend you vote for it, it may be a long time before its added, or never added -
MAC users can already use To-Do on IoS/IPad and the web app at the Web App on https://todo.microsoft.com where you log in with your Office 365 credentials. To-Do has the ability to hide completed tasks.
I hope I have answered your question. If I have please like and set as the solution. If not, please let me know what more you need. Thanks for raising to the community and I look forward to answering more of your questions in the future.