Seeking revenge is very human, but in this case it appears that Superman was being quite careful not to actually kill the guy, just rough him up and give him a taste of his own medicine.
430 CONTINUED
The TRUCKER spins in his seat, uncorks a tremendous right hand to the midsection; he screams as his fist cracks into what seems like solid granite, breaking a bone. CLARK leans down, spins the stool in the opposite direction at dizzying speed. The TRUCKER becomes a whirling blur, his seat spiraling upward until it reaches counter level. CLARK reaches out, taps him at exactly the right moment; the TRUCKER flies down the length of the counter through napkin holders, salt shakers, and other people's meals, crashes against the wall, lands in a heap. The COUNTERMAN gapes at CLARK unbelievingly.
CLARK: (nice smile) I've been working out.
Superman 2: Shooting Script
As to whether this is in character, this is addressed in the DVD Audio Commentary with Pierre Spangler, the film's producer. In short, Superman still has some unresolved issues from the brief time when he was fully human, including a desire to revenge himself.
This wasn't normally in Superman's character, to get revenge. He is a very magnanimous character, but he had been human for a while so he understood the sweetness of revenge.
I suspect the justification would be that this is likely to make the Trucker think twice before bullying the next hapless individual that comes across his path so it's a net win for his victims and for the bully himself, as he may learn to be a better (or at least more careful) person.