Well there is no good answer in my opinion for it.
If you do it, you can be 100% sure the value returned is of type x. Otherwise you depend on the data layer for the right stuff.
I check the values, but that is mostly because I like the defensive programming approuch (everything outside the scope is evil and should not be trusted). The domain object is outside the scope of the mapper, so you are not sure what you get.
Next if you make an api of some kind and use the domain objects again out of scope so check the values.
conclusion it depends on the code style you like. So you could implement the getters and settets or just direct to the values.
Although I recommend use at least getters and setters for the case you need to change how values are handeld (array needs to become object for example)