Exercise Before Tummy Tuck ? Yes Or No ?

Most certainly, yes, exercise before tummy tuck for strength, flexibility, endurance, and your mental well being.  Not just your core, everywhere.  Increasing your lean body mass will provide more protein that your body will mobilize post operatively to support healing.  From a metabolic standpoint, surgery is a stressful event.  You become catabolic for a while.  You will not be able to exercise during this period, so get it in now.

It makes no surgical difference either way.  However, if you are exercising not for enjoyment but specifically in order to prepare your body for a tt, then you don't need to do that.  You're better to just be healthy, then starting 3 months after the tt to work on toning and strength.  You will lose muscle mass and strength during the 3 months after tt as you can't exercise those muscles during that recovery phase, so there's no great benefit to build them up preop.

Good luck with your surgery.  And good question.