Hello there! (I was the translator for this game.) I wanted to point out that the original line in the game includes the swears "kuso" and "chikusho." Yes, Goku uses profanity even in Japanese!
Originally, I had the same opinion as you, Ryusui. I translated "kuso" as "crap," thinking (like you) that Goku was not really the profane type and that strong profanity wasn't appropriate in the game.
Goku is, after all, quite innocent.
After I translate, I let the hackers change things as they see fit... And strengthening the profanity in this instance was one of those things they changed... But when I see the line as they have put it here, I think that it's actually much better than just putting the watered down "crap."
Now shonen manga may be targeted at very young boys in Japan, but a translated NES rom is aimed at a much different audience.
Does this single word destroy the fact that Goku is innocent? I believe that it does not. Goku is not saying it brashly and to sound rebellious. He doesn't even understand that it's supposed to be a "bad word." It's just something that he's heard being said when things get rough. This innocence is similar to the very first manga, when Goku pulls the underwear off of Bulma because she seems to be missing something down there.
Not exactly 9-year-old friendly material in America, though.
This is aimed at a different audience, and I think that they will enjoy the game just the same...
"Gosh! Move it! I said move! Golly!" (Just kidding.)