token orphan

He was certainly aware of their father's doubts concerning them going above ground for the first time because a comment like that doesn't just come out of nowhere.

Granted, this instance was him speaking his mind in the heat of the moment and only realizing how much his words would sting after the fact (especially since they're the ones who convinced Splinter to send them off with their begging), but this becoming a continuous trend of his throughout the series further proves the fact of him being more aware of things than a lot of people pegged him to be.

In Mikey Gets Shellacne, his remark about Splinter telling him not to fret about his appearance when he could hardly bring himself to do the same can be interpreted as a more apparent example of him knowing more than he lets on. It's made rather obvious from the start of the series that Splinter wasn't keen on roaming the streets looking like a giant rat, but that's more of a "show and don't tell" deal where both the audience and the characters are left on their own to pick up on such an important yet minor detail. Splinter never tells anyone about his insecurity, nor does anyone in the series talk about it amongst each other, so a moment like this can be a bit jarring when it's framed as Mikey calling out Splinter's opinion on his own appearance.

Though the intent of his comment was to simply call Splinter old, the underlying insult is there and Mikey regrets saying something like that to their father of all people.

Other than letting his frustrations get the better of him and unintentionally plucking at Splinter's insecurities, Mikey is greatly in tuned with the emotions of those around him and will usually pick up on any changes rather quickly. Take these two moments with, funnily enough, him and Splinter near the start of Serpent Hunt.

I previously posted the first clip as a funny little observation, but the scene is more intriguing than my jestful sentence made it out to be. Mikey's attention is split between hanging his goofy pieces of artwork on the wooden boards and listening to April as she announces how the restaurant is starting to look like a makeshift home. It's not until he turns around to joke with her about his drawings that he finally has Splinter in his sights, and his attention evidently begins to drift to Splinter as April is responding to his question (you can literally see his head following Splinter's movements while April is speaking to him😭). And just look at how taken aback he is by Splinter's melancholic expression being on full display:

While Splinter answers Mikey by stating that he's concerned for Casey, Leo and Raph because they've been gone for a while, it's pretty obvious that's not all he's bothered by, and him not being subtle about his troubled mind like usual had enough flags raising in Mikey's mind for him to hop up from his spot. Once the scene transitions to the second clip, Mikey lingers with Splinter in the front of the restaurant while April ventures to the back in search of Donnie to check on his progress with the retro-mutagen, as well as to see if the others made it back yet. Mikey and Splinter decide to follow April as soon as Donnie utters Karai's name, and Mikey is visibly bracing himself for another look of heartbreak on their father's visage:

Splinter moments aside, Mikey recognizes when something is up with his brothers and friends, as well as whether or not he should step in. Of course the prominent moments of him demonstrating his ability to calm Leatherhead and Raph down count among the times when he steps in, but the times when he chooses not to step in shouldn't be understated either.

Take the pre-intro scene in The Cosmic Ocean for example, when April breaks the silence and questions where Leo ran off to. We understand where Raph is coming from when he says that it can't be healthy for Leo to confide in a simulated version of Splinter since it could just make him miss their father more than ever, but Mikey suddenly pops from his laid back position off-screen and joins the conversation:

He's knows as well as Raph that speaking with a simulation of their father isn't exactly ideal, but he also knows that Leo is used to coming home to confide in Splinter for advice whenever he feels his confidence as a leader beginning to waver. The whole gang knows all of this, but once again, Mikey's the only one to verbalize it. Sometimes people need to hear something so obvious outloud to really understand or remember its importance, and Mikey seems to know that best.

I don't really need to mention how his emotional awareness extends far beyond simply knowing when someone is down, or when he is or isn't in a position to help them, but it makes for a decent epilogue to this analysis so-

Him being the least (read: least) temperamental of his brothers automatically puts him in the position of a mediator when there's tension in the group, and it's common knowledge that he'd often go out of his way to ease that tension in his own ways. He sometimes makes jokes, both corny and intelligent, just to get everyone's minds off of the heaviness of a situation, even if that moment of reprieve only lasts for a minute. He attempts to break up fights before they get too far, which sometimes doesn't work because they're a stubborn bunch of turtles (literally the entire first five minutes of New Girl in Town lol), but his efforts are commendable and genuine.