Messi isn't the only person bearing the blames. Neymar and Mbappe are also doing the same. More blames will be coming because myself I also blame them. Messi's case is different, this is because, at his age, we ain't expecting many extraordinary things from him like He always does when in Barcelona. Secondly, Messi is doing his best to make sure that his younger stars/ Colleagues ( Mbappe and Neymar) receive enough assistance and supports that they deserve from him, with this, much is done by Messi but little will be seen. So as a significant figure, you can't escape such blames.
PSG win and lose as a team and no single person should be blamed for their success or failure in a season. Messi, Neymar, Mbappe are their attacking players who are mostly responsible to score goals for PSG and partially contribute to their winning chance. However they are not god and can not bring PSG to winning in all matches if they don't have support from midfielders and defenders.
In return, midfielders and defenders need support and contributions from those players in defensive efforts which will help them to reduce pressure on their defense system and goalkeeper. Generally, their players must do attacking or defensive task as a whole team not as a single player.
When there are many demands from fans or other parties who have connections with the club, the players will be pressured and their focus will be increasingly broken so that they cannot give the best form of play on the field. The demands from fans due to PSG failure in the Champions League are reasonable because in history they have never won the UCL trophy.
Actually PSG is already at the best point to win the Champions League. The composition of the players they have is above average, but they don't have enough mentality with the atmosphere of the Champions League, when dealing with other elite clubs they often experience defeats which make them have to go home sooner than planned. In the domestic league, there is no doubt about PSG's quality, they always dominate Ligue 1 almost every year, but when they go to the Champions League, PSG doesn't seem to have a winning mentality there.
I think PSG need the support of a quality coach to win the Champions League, like Zidane, Luis Enrique or if necessary they have to hijack Guardiola from Manchester City to make their dreams come true.