Shiryu getting some blood from being smashed in the ground was very much needed otherwise it would shit on everything shown by Garp and other BBPs at the same time.
Garp was using fodders as shields and flinging them everywhere, he sent Wolf in the ocean, he was flinging super huge cannon balls like nothing, he sent Aokiji to the depths. If his slam didn't do anything to Shiryu how bad would it look for everyone.
Compare the stabbing to Zoro getting stabbed by Killer. Zoro and Garp are both physical monsters (Garps hype of punching ships is clear). Zoro got stabbed through shoulder but was strong enough to stop Killer pulling it out and then finish Killer in the very next move. Garp on the other hand was stabbed in the stomach so his shoulder which he needed for the slam was fine (I know core also plays a role in any strength movement but its not the main muscle) and then he proceeded to slam Shiryu who got a little bloodied.