While the idea is a good one, I don't think it'll work. Cruise ships need a lot of money to keep them going - Even bunkers alone can be eye-wateringly expensive. When you then add onto that the cost of food/rations, berthing, crew to sail the ships from their current location to where they're needed... It gets real expensive real fast.
On top of that, you have to factor in that Beiruts port has a very sizeable hole in it, so it'll be very difficult or near impossible to moor the ship alongside the berth. You can't use the container terminal etc., as these facilities are desperately needed to bring in relief and to keep the economy moving. All of this assumes that Beirut is deep enough to accommodate the ships in the first place...
I'd also argue that anything that's already sitting on the beach is a lost cause - Otherwise there's going to be even more costs involved hiring tugs etc. to pull the ships off the beach, then dry docking or at the least an underwater survey to find out what damage the beaching has done to the bottom plates, propellers, rudders, thrusters, etc.
And all of this is without thinking about what to do with all the stuff a cruise ship produces like sewerage etc.!
Personally I'd prefer to see other countries mobilising assistance in the form of aid, both financial and physical, although given the current economic and political climate, plus COVID, I'm doubtful that Beirut will get anywhere near the amount of aid it needs.