Meaning of metrics in Google Cloud Console
It seems that my cluster is capped at 3% of maximum utilization, if I
  read the units correctly. This makes no sense at all, why isn't it
  capable of reaching 100% ?


Why do you think it does make no sense at all? 


  I ran a few tests that created quite a few pods


What is a few tests?


  (each pod running basically an hello world script)


What does this script do? Print helloworld? If that is the case then it could be possible that the cluster will not use 100%.

By the way, how large is the cluster? How many agents? How many memory? So if the cluster possesses 1024GB of memory then it could be the case it is using 3%.

In summary, to indicate whether something makes no sense at all a number of facts is required:


content of the helloworld script
characteristics of the cluster, e.g. totol cpu, memory, type of disks, storage capacity, number of agents, masters
definition of few tests and few pods. How many tests were run, frequency, e.g. 10/s, how many pods


If you would really like to loadtest a k8s cluster, kubernetes has some documentation about this, e.g. this document explains how to load test a k8s cluster and in this case force autoscaling.