Marseille is actually playing very well this season but they still have to improve so they can be better and can be a competitor than Paris Saint Germain at the top of the standings this season they are another class with dance in second place but overall they are playing quite well. Inconsistency is a nightmare for all teams, not just Marseille, Paris Saint Germain also feels the same way.
Marseille are good but not fine in Ligue 1. I mean they are doing good in most of the games played but they missed many opportunities to get closer to PSG and to pass Lens in the ranking. PSG missed too some games but this is just normal, they can't win all of them after all.
Marseille need to accept less goals. Building a better defence line is required. That is the difference between them and Lens. Scoring more goals will make them better than PSG. To obtain that, they need to spend more money

Marseille are in fact a really solid team. Because they can get score contribution from every area in their lineup. Even the defenders have made a nice job about that. When it comes to their strikers Alexis Sanchez did a pretty good work with scoring 14 goals. But there is only one reliable striker in the team now and his contract is about to expire by the end of this month as well.
Marseille had better work on this situation. Even if they manage to extend Sanchez's contract they had better sign another younger and reliable striker beside him as well. This would keep the team very productive even in times of an injury.
Aside from that they need to consolidate their defense line as well. Because that was one of the reason why they were trailing behind Lens this season.