I lived in Liverpool for several years (1991-1999) before moving back to rural East Anglia. I would not describe Liverpool as dangerous, in the sense that you might expect to be the victim of a violent crime.
Nevertheless, I think it is true that there is a higher level of criminality than you might be used to. Car crime is very common and so too are burglaries and general petty theft. What I found hardest to get used to was a general attitude which seemed to regard that kind of criminality as normal and to be expected.
The rule, then, is have good locks on everything and use them. If you rent somewhere to live, it will almost certainly have a burglar alarm, and you should use it. Likewise, make sure you and your possessions are properly insured.
But no, I would still not regard it as dangerous. I suppose muggings do occur, but, while petty theft is commonplace, violence is not.
And it is true: the people are generally friendly, good-natured, and welcoming to strangers. I hope you do enjoy it. It is a place with a great many advantages, including some brilliant museums and art galleries, as well as a great musical tradition, both modern and classical.