Yes it is realistic when they prefer to focus on the Ligue 1. Suffering two defeats made it very difficult for them to qualify from the group stage. Talking about their opponents in this group stage, I can actually say that they are quite evenly matched. But Marseille has not been able to play that well to beat the 3 clubs that are its competitors. Maybe I can say that they are not ready for a higher stage like this Champions League.
To do well in the Champions League, an experienced squad is needed and the players must be confident. Marseille players may be parceled to play well in Ligue 1. But they are not experienced enough. And so we see a poor performance from Marseille in the Champions League. Qualifying to the next round of the Champions League is now almost impossible for them. So Marseille should try to perform well in Ligue 1 matches.
The problem is that the group they are in is also quite strong even though the quality is almost equal. Tottenham are seeded and Sporting CP. On the other hand, Frankfurt are champions in the Europa League, so it will be difficult for them to put on a good performance amid the siege of a team whose player quality is equal or even greater than them.
But even so of course they also can't give up on UCL because they also have to at least get points at the moment as they are Ligue 1 runners up and obviously it would be a joke if they couldn't get points at a higher stage.