Why didn't Admiral Holdo just reveal the escape plan to Poe?
This happens for four major reasons:
Admiral Holdo doesn't trust Poe, given his recent dreadnaught plan.
Upon meeting Holdo, Poe disagrees with her plan without even knowing it.
Vice Admiral Holdo is Poe's ranking officer and doesn't have to answer to him.
According to Leia, Holdo didn't want the attention of being considered a hero.
1. Admiral Holdo doesn't trust Poe, given his recent dreadnaught plan.
Once Vice Admiral Holdo took command of the fleet, and after she introduces herself, Poe approaches her to get clarity as to what the current plan is.
POE: Vice Admiral, Commander Dameron. With our current fuel consumption there's a very limited amount of time that we can stay out of range of those star destroyers.
HOLDO: Very kind of you to make me aware..
POE: We need to shake them, before we can find a new base, so, what's our plan?
HOLDO: Our plan? Captain? Not commander, right? Wasn't it Leia's last official act to demote you, for your dreadnaught plan? Where we lost our entire bombing fleet?
POE: Captain, commander, you can call me whatever you like. I just want to know what's going on.
HOLDO: Of course you do, I understand. I've dealt with plenty of trigger happy fly-boys like you. You're impulsive, dangerous, and the last thing we need right now. So stick to your post, and follow my orders.
From the very beginning, Admiral Holdo expresses contempt for Captain Dameron, with her reasons having to do with Poe's general demeanor, and his actions during the dreadnaught debacle. This is clearly expressed by her characterization of Poe, that being impulsive and dangerous.
With Vice Admiral Holdo being a strategist and Poe being a fighter, Poe and Holdo's philosophies are fundamentally different from each other, and as a result, Vice Admiral Holdo felt that she couldn't trust Poe.
Leia also points this out when she demotes Poe, mentioning that not everything can be solved by hopping into a cockpit and blowing stuff up.
2. Upon meeting Holdo, Poe disagrees with her plan without even knowing it.
When first speaking with Vice Admiral Holdo, Poe expresses that he thinks they should shake the star destroyers before finding a new base. Already Poe has disagreed with Admiral Holdo's plan, and has informed her of it, without him even being aware of it.
If Vice Admiral Holdo already knows that Poe will disagree with her plan, then, why would she feel compelled to tell him, only to cause more conflict?
3. Vice Admiral Holdo is Poe's ranking officer and doesn't have to answer to him.
When introducing himself to Vice Admiral Holdo, Poe uses his previous [higher] rank, but Admiral Holdo makes a point to rectify the inaccuracy. This somewhat deceptive introduction could have rubbed Holdo the wrong way, especially considering that she was already aware of Poe's demotion and the reason behind it. At the end of their conversation, this chain of command is reinforced by Holdo telling Poe to just follow her orders.
Additionally, generally speaking, ranking officers don't take too well to having to answer to subordinates, especially to those who are considered to be disobedient and/or reckless.
4. According to Leia, Holdo didn't want the attention of being considered a hero.
Not only was Vice Admiral Holdo reluctant to sharing the plan with Captain Dameron, she also didn't share the plan with near anyone else.
After Leia and Poe are in one of the escape pods, Leia explains to Poe what Holdo's plan was. When providing a reason as to why Holdo didn't make her plan public, she says,
LEIA: She was more interested in protecting the light, than seeming like a hero.
Vice Admiral Holdo and Captain Dameron didn't really know each other, and, from what little Holdo does know about Poe, she obviously doesn't trust him. From this, even if Poe was told the plan and did agree with it, there's still a great amount of uncertainty on Admiral Holdo's end that Poe wouldn't have told others.
Admiral Holdo and Poe don't have the same kind of relationship that Poe and Leia do, by any means. Poe and Leia seem to have been working together for quite some time now, and, although Leia wasn't happy about Poe's dreadnaught plan either, she still seems to have an amount of trust/respect/tolerance for him. As far as Poe and Holdo go though, Poe had only heard about Admiral Holdo up until then, and, as for Holdo, perhaps she only knows of Poe because of his dreadnaught plan.
Vice Admiral Holdo's plan was, I'm sure, on a need-to-know basis, similarly to how Poe contextualized their plan [Poe, Finn, and Rose's]. Given this, since the plan was evacuation and not dog-fighting, there's no reason for Poe to be briefed on the details, and he should simply wait for orders.
.. she didn't have to reveal it to everyone, just the hot-headed pilot on the verge of mutiny.
The risk of a mutiny didn't occur until after Poe became aware of
the evacuation. And, upon him finding out, Poe got outraged and called Admiral Holdo a coward and a traitor. I don't think these are motivating factors for Admiral Holdo to share her plan.