If you want to determine what the median stat for characters is, then +0 is the starting and +1 is the more then likely soon thereafter. If you want to determine the average, then the average starts at +1 and can reach +2 given the standard playbook. However the "median" stat (discounting the largest two and smallest two) while it might look good, isn't narratively applicable. Since in reality the only way to determine PC competency is to look at the types of moves they make in any given week, and then take the average +stat associated with each move. Which means any stat substitution, any highlighted motivation, and player-to-character choice of problem resolution has far more to do with a determined average then looking at the five numbers.
So if a player asks, whats an average stat? They're really asking a question they haven't determined yet. If you have a hard of +3, and every other is a hard move, and the leftovers alternate between +1 and +0 respectively... over the course of say 20 moves... you have a total stat spread of (3*10)+(1*5)+(0*5) = 35. 35/21 = 1.67. That could mean in this example, a character with a stat spread of +3+1+0+0+0 could have a most common effective stat of 3, and an average effective stat of 1.67 despite the median being 0 and the average being 0.8.
So really, whatever stats a character chooses to roll to resolve a sitch are their average stats, regardless of what moves they employ. The the following three statements provide detail: +0 means you have a slightly better then 50/50 shot, higher means you get your way more, lower means things will spiral out of you control more; +1 is the true and actual standard for badass play-books; and NPCs don't have numbers, they succeed if you don't stop them. They in the latter case meaning, the one that most seems like they'd succeed, when pitted against each other.
Really, even the above math fails to handle this statement well, because when it comes down to character competency, what should be looked at is... What does this character try to achieve, and how often do they succeed in doing so? Best rationalized in the game world by asking, how does this character behave on average, and how often does that behavior get them what they want, rather then backfire?
TDLR;; +1 is always the expected average mathematical competency for any PC not hamstringing themselves intentionally. However the numeric value is hardly a reasonable assessment of skill or behavior within the narrative.
P.S. You could play that same stat spread outlines above... but pass nearly every +0 roll with a 10+ while at the same time failing nearly every +3 roll with t miss. If this randomness continued, then narratively you're not actually above average in hard, you're below, and you're not average in whatever else, you're significantly above. Although if you have a high hard and you try to be hard more often, this might make you look more like that thug who cant fight then a badass who can, even if the rare other rolls are amazing. Just the same, if you never roll hard cause you're trying to be hot or cool instead and we assume your luck continues then you're going to be far above average hottness and coolness.
PSTDRL;; You are what you try to do, not what the stats say. Those just serve as guidelines / probabilities for what situations you might need to play one way to retain your authority. In the end, it just comes down to "What do you do?"