It's more analogous to something you cherish than any real concept.

In real life, you may remember your first game of basketball, or you may not remember something of the sort until you win a big championship game. But there's no biological or emotional guarantee that you'll remember it any better than any other thing you do, even if you really like basketball.

In addition, you'll notice in real life that we're more likely to remember bad events, rather than good. While you might not remember the championship game or the resulting celebration quite as well, you will almost definitely remember the exact position you recall being in when the ball broke your nose during one game. Despite Inside Out's attempt to show "core memories" as a good thing, it's more likely that if they were to have a real life counterpart, it would be trauma rather than anything good, as things which are traumatic for the brain tend to induce more recollection of it.

So long story short: No, core memories aren't really a thing, although you could easily relate it to the way high emotion/trauma tends to induce greater recollections of events. It's one of the many ways movies like this tend to simplify concepts we don't fully understand yet to make a feel good story out of it.