It is a bit of a irony that one must use a chinese service to obtain air quality data throughout Europe. Politicians like to to talk about “open data”, but the progress in providing overarching access to public data still leaves to be desired in many areas. To some extent the data for Germany is provided by the UBA on their web site, but as far as I know they don’t provide APIs to their data services - https://www.umweltbundesamt.de/daten/luftbelastung/aktuelle-luftdaten>. For unknown reasons the UBA web site hides some of the measured values. For example, it is not possible to access PM2,5. Beyond UBA, some federal states run their provide data services, but altogether it is still a patchwork.
Regarding AQICN, my only concern is that it does not provide the actual measurement data for each air pollutant (typically ppm or µg/m³), but it provides an AQI (air quality index) which is not commonly used in Europe as far as I can tell. The AQI has been originally defined by the US EPA and is used as part of the AirNow data service in the US - https://airnow.gov/index.cfm?action=aqibasics.aqi
Note, beyond, AQICN deviates from the US scales in some areas. Thus it is important to lookup the information at their site. See the information on Ozone AQI, for example.
"It always takes longer than you expect, even when you take into account Hofstadter's Law.", Hofstadter's Law