Do you know what model of CD/DVD drive is in the MacMini - is it a SuperDrive or a Combo Drive - the exact model would be good, i.e. CW-8123 or such like.

The optic lens can be cleaned if you are very careful, but you'd need to know if the lens itself has been damaged first of all. For that you really need a good magnifying lens, something like 25x or 60x magnification, as the naked eye cannot see in close enough detail if the lens is damaged or just dirty, as sometimes just dust collected on the lens is sufficient to make it not work properly.

I've cleaned quite a few laser lenses in the past to get them to work as they should, that were thought to be faulty, but sometimes the damage to the lens makes them useless.

You can get quite cheap magnifying lenses on e8ay, ideally with a built-in LED light to see in greater detail/clarity.