Are fat transfers good for marionette lines and droopy neck area?

I would suggest first trying fillers for the marionette lines.  Fat is usually not a good option there.  Also, unclear what you're referring to in the droopy neck but that may be related to skin issues and fat not an option. Discuss with your plastic surgeon.

Yes to both. Both areas mentioned are areas where laxiity occurs secondary to lack of adjacent bone support.Fat grafting on the bone will help support these areas---but like anything else only to an extent.  If too much laxity and eventually too much descent of overlying tissues has already occured you may require concomitant soft tissue tightening or perhaps even a lift. Only an in-person, detailed examination can make that determination.Hope this helps.