Is stop leak bad for your car?

No, it is not bad for your car; most stop leak also includes anti rusting or lubricating agents; but it is also not designed to be a permanent fix. If you need stop leak for either your coolant or oil system, there is obviously some other problem that needs attention, (gaskets/seals) and really should be taken care of within a few months time, even if the stop leak cures the problem. In some cases, the size of the original leak may have a lot to do with the course of action on the repair. In coolant systems, a small leak MIGHT be repaired permanently with a stop leak that contains what looks like tiny balls of solder.

Yes, it can be. Radiator stop leak can plug the passages in the radiator, heater core, or engine block.