The word "wow" generally only indicates that the speaker is surprised by something, it doesn't imply anything good or bad about how the speaker feels. Upon hearing good news, you could say "Wow! That's great!" and upon hearing bad news you could say "Wow! That's terrible!" In either case, the "wow" is just for shock value, and what follows describes how you actually feel about the news.
In the specific case of this quote, Yoshi is simply expressing surprise that Seymour has just been fired. Saying "wow" in this case doesn't mean that he's happy Seymour got fired, it just means he couldn't think of anything more relevant to say.