According to Google Ads, "Performance Max is a campaign type that provides a unified buying service to all Google Ads inventory. It helps advertisers optimize performance, in part by reducing the complexity and overhead needed to manage multiple campaign types. Performance Max campaigns allocate budgets dynamically to the highest performing channels, including YouTube, Display, Search, Discover, Gmail, and Maps, eliminating the need for advertisers to update budgets across multiple campaigns."
ChannelMix does provide reporting for your Performance Max Campaigns, but it is only available at the campaign level. This limited reporting is due to the design of the campaigns themselves in Google Ads. The API limits the availability of this data to only the campaign level, because when a Performance Max Campaign is created, you create a campaign with no ad groups and asset groups across all ad formats. Google then decides which channel should be used to serve your ads for you.
Here is response from Google Ads team on an API Forum (link at page end): "Kindly note that Google Ads API supports performance max report data at campaign level, and not ad group level or ad level. This is working as expected if you’re getting empty data at ad group or ad level reporting. Note that the performance (impressions, cost, clicks, etc) for PERFORMACE_MAX is available using campaign resource. Also, please make sure to not select an AdGroup when querying Performance Max campaigns, as no campaigns will be returned."
NOTE: The network, advertising channel type, and advertising channel sub-type fields will usually populate as "MIXED" for Performance Max Campaigns in your base datasets. We have adjusted the 'channel' value in the OneViews to populate "Display" for these campaigns.
ChannelMix includes campaign level data in the below OneViews in order to provide you with the ability to report on these types of campaigns: