There's no harm in paying and then appealing, based on a lack of visibility.
I'm not sure there's much use in us arguing as to whether that appeal would win; I think it's clear that it depends on what is understood as 'readily visible' and to me from a common sense perspective that sign - if it is the only one passed - looks marginal, in that I could believe it could be called either way.
But pay, appeal, see what happens.
That post by cuccir makes perfect sense.
There are a number of factors that need to be considered in hoping that any appeal might be successful, but I'd ask the OP to be aware of the following
Firstly, I suggest that it is your daughter who must appeal, not you. It looks from your original post that your daughter is over 18 and notwithstanding any personal arrangement by which you may make the payment on her behalf, she seems to be an adult.
The need to buy a ticket, the need to respond to a penalty notice and the responsibility for any 'alleged offence' are hers.
Quite often, parents trying to take over in an understandable and well-meaning wish to help or protect their offspring, but this can cause delay and diversion in getting these things resolved quickly because they cannot give a first hand account of what occurred, whereas the Authorised Collector, who issued the notice can do.
On the subject of visibility of a notice, it is universally accepted by all promotional planners that the most prominent and eye-catching colour combination is clear black text on a bright yellow background. It's the reason why all speed cameras are painted bright yellow.
The Penalty Fare notices use that combination for exactly that reason.
I agree that coming from the direction that the photographer is facing in that particular view of Poynton, it is not immediately easy to read without slowing down and looking at it for a few seconds, but it is as prominent at or near average eye-level as can be on a wall alongside the walker and the bright yellow is fairly obvious.
What would be more important in deciding whether any appeal could turn on just that visibility point, is confirmation that there is no other signage on the station. As Killingworth has kindly provided images of all Northern signage in post 8, it is clear that the signs do meet the requisite wording requirement. The constant 'grasping at straws' by some posters can give those seeking help unrealistic hope.
For example, what other signs are displayed? What signs are situated at or near to the TVM at Poynton?
It is important for TOC and traveller to abide by the National Rail Conditions of Travel, but it is not NRCoT that a traveller would be prosecuted under.
National Railway Byelaw 18.1 [2005] makes it a strict liability requirement for every traveller to obtain a valid ticket before boarding any train at a station at which all the facilities to do so are provided. The facility and clear instruction as to what is and will be accepted seem to be in place at Poynton.
If I were the OPs daughter I would certainly pay the 20.00 penalty and consider whether any appeal is worthwhile, but I would not risk the likelihood of a fine & costs that will run to hundreds of pounds as an alternative to paying that small sum now.