No, don't put the bikini on. You don't know where that's been, and the last thing you want is to expose yourself to whatever "fun" monster-borne infection could be lurking on that thing. Besides, despite what trashy fantasy novels have taught us, bikinis are NOT effective combat gear.
See if you can open up some of the other lockers instead. Maybe they've got something more useful inside.
Why would you put it on? You are in a creepy theme park with monsters lurking around. What are you gonna do? Invite them for a splash? However it might come in handy for chocking Marissa if you see her. Take it with you.
It IS a sexy bikini-- check the size, to see if it would be sexy on YOU, or if you've basically just found some fabric you can use for a sling or bandages or a slingshot or not leaving fingerprints in incriminating locations.