First let's cement the difference between shameful and shameless, I'll just use whatever google gives me as I did with "prude" earlier
worthy of or causing shame or disgrace.
(of a person or their conduct) characterized by or showing a lack of shame; barefaced or brazen.
So while an action can be shameful, it cannot be shameless if the subject expresses shame as a result.
If the subject recognises the action as shameful, their intention is to perform the shameful action, and they do not express shame, they are wilfully shameless in that instance and could reasonably be considered a shameless character.
If the subject does not recognise the action as shameful, they cannot feel shame, and we could label them as shameless in this instance but it's not really a fair judgement of character. Examples would be accidentally showing your underwear or posting an image without noticing its shameful implications.
If the subject recognises their action as shameful, but the action is auxiliary to their intention, we would reasonably judge their intention. Example maybe removing your trousers in order to plug a sinking ship. Again we could label them shameless but it's not particularly meaningful as a character judgement.
We began with a mousemat that depicts her accentuated buttocks and panties. Assuming that you're not socially and/or sexually retarded, you recognise why this might be considered shameful, there was no secondary motive, and shame was not expressed. This was wilful shamelessness and we might reasonably judge you as a shameless character. You then claimed that Ruri is "just as shameless" implying that she is also wilfully shameless in the same way. This is what I have been arguing, firstly because it's the point that was originally made and secondly because any other interpretation of shamelessness is largely meaningless.