It may be natural for a domestic cat to be fearful of visitors because visitors enter the cat’s home range and may intrude on the most ‘intimate’ parts of their home range (the room where they feel most comfortable). They are intruders and as they are much larger than cats the default mentality is defensiveness and varying degrees of being frightened.
Common sense says that domestic cats have a range of confidence levels. Cats who are scared of visitors are generally timid. Jackson Galaxy calls them ‘wallflowers’. It is not all bad for wallflower cats because their lives are safer as they avoid hazards. Also they probably make better indoor cats which is a big benefit.
However, being a wallflower cat means that the cat does not live as full as life as she or he deserves. Cat guardians probably should not accept and reinforce overly timid behaviour in their cat. But neither should they try and force it out of them. Once again it is about patience.
The reasons for timidity/fearfulness in domestic cats can normally be put down to (1) a lack of proper socialisation (2) a threatening environment provided by their owner or previous owner(s) and (3) a genetically inherited character trait.
The way to remove at least some of their fear of visitors is to make them more confident and therefore assertive in front of visitors. Referring to the first of the three causes for timidity above you can make progress on socialisation by carrying on the socializing process that was begun in the first 8 weeks of the cat’s life. I don’t think you have to decide if she is under-socialised because the treatment for all three is essentially the same. I socialised by feral kitten through a lot of play, good food and presenting as a very friendly and reliable human animal.
It means getting her to make her mind and movements more open. Her territory should be expanded if she is retreating to a safe bolt hole. One good way to achieve this is to play a lot more with her than you might otherwise do as it distracts a domestic cat into forgetting her timidity because instincts take over. Cats who are scared of strange people may play with a tease held by a strange person for instance. When they realise the tease is held by a visitor they lose some of their fear of them.
Making all parts of their home cat friendly can help a timid cat to become braver in exploring new areas. The environment should be made as cat friendly as possible with vertical spaces for example. All the usual conditions apply such as calm, routines, reduced noise.
Once the cat has gained a bit more confidence, it may be practical to ask visitors to not use the doorbell. I don’t know about you but my cat always dives for cover when the doorbell rings and he is not a particularly timid cat. If you can meet a visitor at the door and let them in it would avoid that initial sense of fear and the connection between strange people and the sound of the door bell.
It is better if visitors do not try and interact with your cat. Let the cat get used to the visitor’s presence. It may take many visits by the same visitor before the cat adjusts and accepts his presence.
Simultaneous touching and talking
I find that if I am touching and talking to my cat while the visitor approaches with an outstretched hand (no pointed fingers – the back of the hand is better or downward pointing fingers) his natural inquisitiveness accepts the strangers contact (with a sniff) and from their he will begin accept a gentle stroke in due course.
Gift of a toy or treat
Thirdly a visitor can bring a present for your cat. This might be an effective toy. Some modern toys are too clever and complicated and can scare cats if they make noises and are too bright. The toy should be a simple tease. If the cat’s desire to play overcomes his nervous caution he’ll be on the way to overcoming his fear of that visitor.
Because both timidity and a default dislike of visitors as referred to in the opening paragraph is within the range of normal behaviours, I don’t think that it is fair to consider it wrong or bad for a cat to be scared of visitors. It is more normal than abnormal in my view. Therefore the issue should be accepted with good grace and changes made gradually and sensitively.
Other experienced cat owners will have equally or more valid thoughts. Please share them as they are very welcome.