Whether a lie will be believable or not is what makes the decision uncertain.
Not by 5e RAW. Uncertainty is the player or DM not knowing which way the NPC or PC will go. If the DM or player does know which way the PC or NPC will go, even if it results in the wrong answer or decision, the outcome was never uncertain. I the player am free to decide whether my PC believes or doesn't believe the NPC, regardless of how well or crappy the NPC is at his lying ability.
Earlier this year I was running my game and the PC druid was talking to an NPC. The NPC tried to lie to him, but I knew the NPC was really bad at lying. There was no meaningful consequence involved, so I simply told the PC that he noticed the NPC fidgeting nervously and not making eye contact as he spoke, and that he could tell the NPC was lying. The player corrected me, saying that his PC having lived his entire life in the wild not only doesn't know about that sort of body language, but really doesn't even care, so he doesn't pick up on the lie. Immediately I was like, "Okay. You don't notice the lie." and we kept playing with the PC believing what that NPC said.
Not exactly, it's when the DM doesn't know whether the PC/NPC will accomplish something or not: "If the total equals or exceeds the DC, the ability check is a success--the creature overcomes the challenge at hand. Otherwise, it's a failure, which means the character or monster makes no progress toward the objective or makes progress combined with a setback determined by the DM."
So, in the case of deception, will the PC/NPC overcome the difficulty of producing a convincing lie in the circumstances.
When it comes to things like intimidation and persuasion, though, only the player can determine uncertainty.
"The DM calls for an ability check when a character or monster attempts an action (other than an attack) that has a chance of failure."
The key word there is "chance." When it has a chance of failure, which also means a chance of success(ie uncertain), a roll is called for. As a player I know 100% whether an NPC can persuade or intimidate my PC. There's no "chance" involved, so no roll can be involved.
If the NPC wants to talk my PC into taking him with me as I journey to the next town, I will make that decision without a roll. Once I've heard his reasons and arguments for why I should take him, I'm going to make that decision. If I say no, there's no roll that can make that change. no, means no. The only way to change it is to come up with a new reason for me to take him. I will then decide again based on that. Maybe I will say yes based on this new information. Maybe I will say no. At no time, though, is the outcome in doubt. It's either absolutely yes or absolutely no, which means no roll is involved.