Yes, Ljubljana is one of the safest cities in Europe.
Like most European capitals, there are pickpocketing and scams here and there, so you should remain vigilant all the time. But it is generally safe.
Just remember the following:
Always keep an eye on your valuables.
Don’t accept anything from strangers.
When in doubt, ask your guide or hotel staff for advice.
Scams are NOT as rampant in Slovenia as in other touristy destinations in Europe, but we still recommend reading these posts:
 HOW TO AVOID SCAMS IN EUROPE
 HOW TO AVOID PICKPOCKETS IN EUROPE