When Two-Bit and Pony visit Johnny in the hospital, he is near death and partially paralyzed. Considering Johnny’s troubles, why do you think that the first thing he complains about is not getting enough hair grease?

From the text, we can infer that Johnny is trying to lighten up the situation.... and the visit.

Two-Bit looked around. "They treatin' you okay, kid?"
"Don't..." ---Johnny gasped--- "don't let me put enough grease on my hair."

He's trying to bring a charming spirit within the room because Two-Bit and Pony were very concerned that Johnny was not going to live so then with Johnny bringing the fact about the Hair Grease, it ligtened the mood within the room from the sadness that Two-Bit and Pony were feeling from the way Johnny is.