I don't know if Christians read Genesis the same way, but the traditional Jewish interpretation is that God forbade meat-eating until after the Flood. Cf. Genesis 1:29 with Genesis 9:3. So yes, Noah was a vegetarian until after he came off the ark.
Doesn't make sense, though. Historically, you see cave paintings that show humans hunting. Long before the Genesis story came out.
Not to mention that according to anthropologists, it made sense that pre-historic people ate meat. High density food that sits in your stomach for a long time.
If all we ate were plants, we'd be hungry all the time, and also be LOOKING for plants all the time.
That's not efficient.
Any survivalist will tell you that if you expend all your energy trying to find food, once it becomes a greater effort to obtain food, than the benefit of eating it, your body says "no more" and you give up.
Not saying Genesis is "wrong" about this. It just doesn't add up with what we know historically.