I'm not sure what is considered too hot for a case temperature. If it's 60C that's about 140F. Personally, I think it's on the warm side. A better indicator is to monitor internal temperatures of the CPU, GPU, hard drive, etc. There are several free programs available. Two that are recommeded frequently are Speccy and SpeedFan. Don't know if they're compatible with an iMac, however. Speccy - System Information - Free Download SpeedFan - Access temperature sensor in your computer
This article goes into more detail about maintaining your computer. It would also help if you report back what the actual temps are for CPU, etc. How to clean inside your computer case safely | Computer Tips EDIT:
Just re-read your question and noticed you're using an iMac. Don't know if Speccy or SpeedFan will be compatible but the general cleaning and care info will be.