Joke aside, I thought the artwork was pretty good.
Petrie and Cera look a little cartoonier than their usual style, especially in the second and third panels (though perhaps that was intentional?), but they're still very recognizable, and the style works well. Also, if this is the first time you've ever drawn these characters, then you did a darned good job!
I hope you don't mind, but I'm going to try to identify what was wrong with the joke:
The original second panel (which I did not see) simply had Petrie state that Cera would grow another horn when she was older, right? I think the problem might have been that because Cera presently has only one horn, people were confused because they thought Petrie meant that she would grow one other horn, and thought both "but she's going to grow TWO more horns!" and "why would she get freaked out by being told that?".
Also, in the revised version, Petrie is making a hypothetical statement; he said that
if Cera grew an extra horn, she would be a "quadhorn"; he didn't imply that she that she
would. And while it's less of a significant issue in this version, he still didn't specify that he was talking about a
fourth horn growing.
I hope that's in some way helpful. Don't give up on making LBT joke pictures; just try to look at the punchlines from the perspective of someone who's never seen them before, and make sure that they would make sense.
