You say you are 'not guessing' about faking depression and other things, but then you say that you 'think' her chronic pain is made up.
Sounds like you are guessing to me.
I work with people that have chronic pain, and have to deal with people like you on a daily basis. People that believe if it's not visible it doesn't exist. People that think if someone is happy and smiling then they are clearly faking being in constant pain. People that think if you can walk ok then it can't be as bad as all that.
We live in a society that has lost empathy. I have even had someone in HR tell me about a colleague that was clearly taking the piss with the amount of time off work. This person clearly had not taken the time or made the effort to find out the history of this colleague and had very much taken it on face value that because the person in chronic pain wasn't in a constant grump about it, they were obviously just lazy.
Now I get that there are some people that take the piss and make a big deal about nothing. And your neighbour might be one of them. But you you have no way of knowing the level of pain she is in - and in my experience you wouldn't recognise the majority of people in chronic pain as being so.
Your neighbours pain is you neighbours pain. Whether it is physical, psychological or (as you suspect) made up, there is clearly an issue. People don't make up chronic pain unless there is an underlying issue - no matter what that issue is.
You don't like your neighbour. That's ok. But that does not give you the right to make her life more difficult. Would it be ok if she made your life more difficult if she doesn't like you?
Your acquaintance can make up her own mind about your neighbour. Or you can gossip and your acquaintance can make up her own mind about you.