Alistar is a very strong early game champion because alistar had a very high auto attack in the early game and if you build full ap you can do alot of damage in the early game also with his wq combo and alistar's passive makes alistar a strongest champion in the early game.

once alistar is level 6 you can actully tank the turret if the enemy has a low health because with your wq combo and auto attack you can actully do so much damage to the enemy champion.

Alistar aren't really good at farming in the early game, because if you are going against a range attack champion they will just poke you really hard in the early game.
However if you are going against a close combat champion it going to be easy because you can farm safely and also if he try to poke you, alistar can actully fight against the enemy in the early game because you had a very high auto attack.