Paulo, based on the idea of "coming out wrong" or "sounding bad", I'm gonna say that smelling fishy is not the right phrase here.
Smelling fishy is more like for a suspicious or dishonest situation, like a scam or something. "To sound bad" or "to come out wrong" indicates that whatever is being said does not conform to common morals or common sense, whether in actual fact or only on the surface.
"This is gonna sound pretty bad, but I really had no idea that what Trump was proposing would affect you that way."
This usually does sound bad, because you would normally expect someone (in the US) to be paying some attention to the national politics of the day, especially if it was a policy issue that would affect some of their friends.
"I hope this doesn't come out wrong, but I've always thought that the South had at least some political legitimacy to their cause in the Civil War."
This could sound very weird/wrong in today's political climate, race-relations being what they are, but the person saying this might have very detailed reasoning behind what they believe, so they preface their introduction with a hope that it doesn't come out wrong, so that people might not get angry/confused right away, and might pause overly reactionary responses until they've at least listened to the idea.
So the suggestion for a literal translation of the title might be: "I know that this is going to/gonna sound bad..."
And one could also use: "This might come out wrong, but...", "I hope this doesn't come out wrong, but...", "I know this is going to/gonna come out wrong, but..."https://www.google.com/search?q="this+is+gonna+sound+bad"https://www.google.com/search?&q="This+might+come+out+wrong"https://www.google.com/search?q="I+hope+this+doesn%27t+come+out+wrong"