"Bad" is probably a strong word. But issues are:

1. Reliance on Google Docs' uptime. While any monitoring service will in theory present the same problem, you're putting all your eggs in one basket.

2. It's just the wrong tool for the job. Install and learn Nagios or Zabbix or what-have-you. If you're worried about uptime, get a couple VPSes from different providers and run your monitors from both.

There's nothing utterly horrifying about it technically, but I just can't imagine how one comes up with this answer to the question of monitoring services.