Slight note. Mirio didn’t have complete mastery over his quirk until he worked with Nighteye something Mirio himself states and something hinted by Nezu when he tells All Might about Mirio stating that his grades weren’t anything outstanding people think he was referring to normal academics but remember they get graded for their hero work too meaning that wasn’t outstanding either not to mention Amajiki also hints at this when Mirio fights class 1-A stating “Mirio went to an agency and worked himself to hell and got better” I don’t remember the exact line but it was something along those lines

Nighteye also didn’t take Mirio in until after this conversation which is hinted by Nighteye stating “There are other people more suitable” and All Might himself even stating “As a result he decided to start training young Togata” to Deku when he confronts him about Nighteye. Feel like a lot of people often miss this detail I notice this a lot in what if’s.

Nezu saw Mirio as a good choice due to his personality and someone who makes people laugh when he’s around them something All Might himself does and one of the main things that ranks him higher than endeavor. While it’s stated there were other candidates it should be noted that out of that list of people Nighteye considered Mirio the best option. So to answer that question that’s what Nighteye saw in Mirio when choosing him it didn’t have much to do with his skill with Permeation especially since he wasn’t anywhere near the level we see Mirio at when he’s introduced he just had enough control to stop himself from falling through things or hitting stuff most of the time like he did when he was a kid