PythonAnywhere dev here: There isn't, unfortunately, but that's a great suggestion! I'll add it to our list.
Update
And here is new (2019-09-11) API endpoint:
/api/v0/user/{username}/cpu/
GET Returns information about cpu usage in json format:
{
"daily_cpu_limit_seconds": <int>,
"next_reset_time": <isoformat>,
"daily_cpu_total_usage_seconds": <float>
}