It's a bumper sticker for God's sake not a philosphical thesis. The point it makes is fair (and funny) enough even if it isn't an entirely water-tight argument.
Some pro-lifers might find it "fair and funny," but I think that pro-aborts are just laughing at us....not with us on that.
Personally, I'd prefer, "SMILE: Your mom chose life." It makes the point, and it is water-tight. No one alive can argue with that.