As noted, it's great, and it doubles down on what Dexter wants to be doing every round- putting assets into play. Bursty guns & swords become very appealing, giving Dex something to do with his hands.
The problem, as with many signatures, is reliability. You can't really plan on it showing up, as to my knowledge, Dexter has no native way to tutor it... except for ... his OTHER signature option.
Intentional? Encouraging "Not only are you allowed to use both sets of signatures on an investigator: in this case, you should!"?
...eh, likely not. Sure, this is a nice card- and so is Molly- and Molly herself CAN be tutored with limited reliability- but Renfield Probably -> Calling In Favors -> Molly -> Showmanship seems like a long road that's interesting but inefficient.
Something of a tangent: it really feels like taking an additional sig + weakness is a trap. For 1 card, you're thickening your deck AND adding an additional weakness you didn't have to take, and even mild weaknesses like Dexter's are a major tempo loss at best. If you think about it, many would leap at the reliability of forfeiting their sig in exchange for losing the accompanying weakness. Not just the Rolands & Mandys of the world who's bad decisions are so strong you have to build your deck around them. (In fairness, this is sometimes to balance the char's inherent skills- this seems the case with Mandy's weakness.) An optional card has to pull a LOT of weight to be worth "Add a weakness to your deck".
Back to Showmanship itself, it's fine. Either it or Molly are solid; Molly adds consistency, Showmanship leaves your Ally slot open for more asset-cycling. I like the limited-synergy they have together. It would be neat to see this go further on a future 'gator who had replacement sigs explicitly designed to make the option of taking ALL sigs at once a genuine temptation.