Well in the Champions League he has scored a couple of goals so far. In Ligue 1 it's definitely not going well for him. If I were Pochettino I would also leave Ramons on the bench. He doesn't seem to be at top level anymore. Way too bus with things that happen outside the pitch. Crazy that PSG decided to pay him such a high salary although they knew that Ramos is frequently injured and misses most of the games these days.
From my point of view PSG signed Ramos so as to add his wealth of experience into club defence, UCL trophy is the main target for PSG in order to win that coveted trophy they need to improve on their defence line and with the signing of Ramos that area of the team would have been taken care of, it is not compulsory Pochetino plays Ramos in every league-1 matches, which I agreed with you, of course PSG has dominance in that league but their main objective and aim is winning UCL this season thus has to fix their team in every department once they achieve that they might sell him.
Yes I get that and usually the transfer could have made a lot of sense, but if you look at the games Ramos played for Real Madrid in his last season, he was really injured most of the time and when he played he wasn't very good as he was out of shape. What were the odds that he would fully recover and get back to old strength? He is going to turn 36 in 3 months. It is not like he had one single injury and could fully recover after some time. He had injury after injury, a very obvious sign for someone who has done the job but won't get back to top level anymore.