They are considered "midcore" because the only thing holding a player back is typically time-investment, gil, and accumulating whatever resources the relic wants.
The only difference between a casual player, a midcore, and a hardcore is how quickly they complete their given relic and how many get completed. A hardcore player can be expected to complete a relic for each and every class. Meanwhile a casual player will take far longer because they are not as deeply attached as to doing it. So midcore are the mix of the two where they may not care about collecting every relic, but they will care enough to collect what relics they actually want for the handful of jobs they actually play. A midcore may even only want one whole weapon and once they are done, they don't care about the rest of the stuff associated with this relic grind.
Midcore are even weirder because they can be really hardcore about specific aspects of a piece of content, but also simultaneously are casual about content within the same sphere. They might have completed P8S, but then for the next tier they can't be bothered to do it. They might do hardcore content, but its only because it has a title or mount or weapon that they actually want.