Sit-Ups After Surgery

I'd like to firm up my "spare-tire" area. Will sit-ups help

No - exercises don't target fat loss. For an explanation, Google "spot reduction myth".
Personally i'd focus on an overall calorie deficit. For exercise, cardio and full-body strength training is the usual advice.. assuming your doctor clears you for it.

sit-ups don't burn abdominal fat................ a calorie deficit burns fat.... exercise and adequate protein intake helps to maintain lean mass while eating at a deficit to lose fat

check with a doctor about what exercises you can do and when.

I've had 2 c-sections (major abdominal surgery) and a minor abdominal surgery.... recovery from c-section was around 3 months and from the other one was a month or two, albeit that the doc read me the riot act re what would happen if I tried to lift heavy weights too soon. But she gave me the all-clear to lift again sooner than she'd originally said, because I healed up so well

you're probably fine to do any exercise now since the surgery was back in November, but you *must* get advice from a doctor, to be sure that you've healed up well and also in case any particular exercise can cause a relapse of the hernia or any other complications.