I've been told before that my hair was too long to "generate the required respect". Old fashioned views from old school observers, but those opinions definitely do exist among the people who will be grading your progress. Those opinions will die out over time, but there's no way to guarantee they'll be gone from the game by the time it will matter to you. And the same applies to fans watching - John Moss clearly passes the FA fitness test every year, but it doesn't stop him being slated online for looking a bit fat compared to the players he's refereeing.
One thing I will say is that you won't be able to get anything football-related (or that could be misconstrued as football-related) anywhere visible. If your sleeve design has birds in them, better make sure that bird doesn't look anything like a Liverbird or the Brighton seagull. No Wolves. A devil? That could be seen as a Man U reference. Any numbers or dates? Make sure a team didn't also win a big trophy on that date or in that year.
One thing that has always been true and will always remain is that the FA are cautious. If they have two equally good SG1 candidates, but one has arm tattoos that miiiiight be a reference to support for Chelsea and the other has bare arms, you don't need me to tell you which one they'll pick.