The qualities expressed by an antihero are opposite of those expressed by a true hero. Where a true hero is brave and resourceful, an antihero (also spelled anti-hero) is weak and limited. Where a hero is personable and honest, an antihero may be annoying and manipulative. A hero succeeds in his quest through virtue and valor and the help of friends. An antihero may succeed or he may fail. He is often alone but, when he has friends, his friends are of the ignoble sort. The list of qualities and antithetical qualities goes on, but this gives a foundation for examining Meursault.
Meursault annoys people: "I had an idea [my employer] looked annoyed.". Meursault is not personable: "But maybe that’s why one day I’ll come to hate you." Meursault is weak, which is part of why he comes to be condemned on trial, and has no resources--he killed a man "because of the sun." He has no virtue: he agreed to write Raymond's letter knowing the purport and intent of it. He has no valor: "but I spoke too quickly and ran my words into each other. I was ... nonsensical, ...."
In one sense, he fails at his task in that he is convicted of the crime he commits. It may be argued that in another very different sense he succeeds in his task because he proves with his life that the world that he sees is senseless and without meaning--that the only things that matter, either for good or for ill, are physical sensations: "I explained that my physical condition at any given moment often influenced my feelings." Yes, Meursault fits the definition of an antihero.