Nope, Law is a realist, not a coward. Nor is he hiding, he is looking for something.
Also, when Luffy finally fight Big Mom, he will not be alone, rest of his crew will be there to fight Big Mom's crew. Right now, we don't see Law's crew. Add their absence with Law making a comment that he wants back too, we can assume they are not simply hiding but are being kept away from Law.
Law is alone right now with no crew.
Since you've compared him to Luffy lets go with that. Lets imagine that some theories put forth here are correct, Law's crew was taken by a yonkou and he wants them back. That would be comparable to when Luffy wanted to save Ace. Luffy could not do that alone no matter how hard he tried. Luffy accepted the help of others in Impel Down and fought beside Whitebeard & allies at Marineford War. Luffy had to learn the hard way from those experiences that to go against someone so strong without backup to take care of the middlemen is suicide.
Unlike Luffy, Law actually thinks about situations and plans for the future instead of just going with the flow as Luffy tends to do. That does not make make him a coward, it makes him smart.
I'll restate my theory that Law's crew isn't even alive.
He wasn't talking about body swap when mentioning Luffy and getting something back, I think he meant Ace's death.
We only saw four characters in his crew who even have names, two of them haven't shown their faces, one joined at Sabaody, and one is a bear for crying out loud. They were fodder to begin with.