Seems fine to me, as long as that's not their regular diet.
There's a lot of lists out there about all the terrible junk foods you're not supposed to feed chickens. Funny thing is, 90% of what's on those lists could also apply to people. Greasy salty pizza is not great for anyone, but as an occasional treat it's not likely to do anything worse than give them a bit of indigestion.
Most of the really bad foods - the ones that people online warn you will "poison" your birds - the birds are smart enough not to eat. Things like milkweed, avocado pits, onions, etc, they will instinctively avoid. The only human "food" - though I wouldn't call it that - that I really avoid giving them is the really high-sugar super processed stuff like cake and candy, which we don't really eat in my house anyway. That stuff is so far from anything in nature that their instincts won't recognize it as the poison it is (it really is for us, too).
But pizza is basically bread, cheese, and tomato sauce, and the only stuff that's really bad for them is the excessive salt and processed oils, which I think are a small enough percentage of the total weight of the food to be no more than an irritant, although I wouldn't give them lage amounts of it regularly.
Isn't it funny how people will feed their cats and dogs cheap brands of food that's basically the most low-grade meat that's considered unfit for human consumption, processed with tons of junk to make it taste better, just because it comes in a can that says "fancy feast"; and they'll feed themselves ding dongs and hot pockets and glucose syrup drinks as long as they come from a nice clean package; but when it comes to their chickens, they'll insist that anything less than organic formulated layer feed and fresh veggie scraps from a list of approved foods will poison them.
