Attack the fantasy decision, not the person
'Starting that running back was criminal' is usually safer than insulting somebody's intelligence, appearance, family or real-life problems.
Use callbacks
Great league humor comes from history: the trade somebody still regrets, the championship collapse, the player one manager refuses to drop. Shared references feel personal without being cruel.
Know the audience
People have different tolerances. If a joke makes someone defensive instead of competitive, it did not work. Adjust.
Let the loser laugh too
The gold standard is trash talk the target would repeat to someone else. If only the winner enjoys it, the joke probably needs work.