As we already established, Sansa is much smarter than Jon. She knew not to underestimate Ramsay, she was smart enough to know that regardless of how the battle went, her brother Rickon was as good as dead the second the Umbers turned him over to the Boltons, and she knew that the army she and Jon had amassed was not big enough to defeat Ramsay's. Women like Sansa are the true heroes of Game of Thrones, but that doesn't mean they're free from criticism.
In terms of what Sansa did last night, it was a temporary stroke of genius getting Littlefinger to devote the Vale's army to her and Jon's cause—and it made all the difference. But someone needs to explain to me why Sansa felt it was a good decision to not tell ANYONE that thousands more soldiers may or may not be on the way. That's pretty useful information, right? Like, the kind of information that might've caused Jon to press pause on the whole thing, saving thousands of lives in the process?
Did Sansa think Jon would turn away Littlefinger's help if he knew it was coming? Did she not want to make a promise she wasn't sure she'd be able to keep? Did she just really enjoy the end of The Two Towers, and want to recreate that Gandalf scene for herself? Whatever the answer, keeping those reinforcements a secret was selfish at worst, stupid at best.
It also speaks to Sansa's shortsightedness. Her war is far different from Jon's—he wants to defeat White Walkers and save mankind, she pretty much just wanted to exact revenge on Ramsay. In stopping at nothing to accomplish that, Sansa once again tied her fate to Lord Petyr Baelish, the man who's partially responsible for getting her dad decapitated. In her mind, bringing the Vale to the Battle of the Bastards is repayment for Littlefinger marrying Sansa off to the most sadistic man in Westeros, but she should know better. Littlefinger doesn't do repayment—he only does things that benefit himself. And now that he's helped Sansa get what she wants, you better believe he's going to be expecting something from her in return.
Verdict: Only a little dumb—but very misguided