Yes it's safe if you use common sense. Avoid dark alleys and control your belongings to discourage pickpockets. Marseille is a genuine working city, not some fancy place like you find further east along the riviera.

I'd suggest a couple days in Marseille for relaxed sightseeing and good seafood. There are hills, but this is a walking city with a small but easy to use subway system. In addition to a fancy meal at Le Petit Nice, go to one of the simpler places around the old port.

If you have more time, I would go to Aix-en-Provence for a day. There
should be frequent train service, perhaps at most a hour long, or you could move to Aix for your last night as it's closer to the airport than Marseille is.

Cassis and Bandol are along the coast to the east, but realistically you would want a car for these places.