M!LK's "イイじゃん" is a pep talk set to a bouncy beat, and the title does most of the work. イイじゃん is casual Japanese for "that's great" or "isn't that nice," the kind of thing a friend says when you show them your new haircut.
The verses are sweet and a little earnest. The singer promises "どんな時も味方でいるよ" (I'll be on your side no matter what) and tells you "誰かの為に生きないで" (don't live for someone else). He even makes room for bad days: "俯いた顔 その涙も 唯一無二さ" (your downcast face, even those tears, are one of a kind). There's a small jab at the times, too, an age where "何もかもがレッテル貼られる" (everything gets a label stuck on it). レッテル comes from the Dutch word for a label, and here it means being pigeonholed.
Then the chorus turns into pure hype, and it's packed with slang worth learning:
The second half gets more personal. "最近自分を好きになれてきた" (lately I've started to like myself) is a nice admission from a group singing to fans, and "画面の奥 見つめている" (staring out from behind the screen) feels like a nod to people watching them on their phones. The last choruses swap looks for nerve: "夢追ってイイじゃん" (it's fine to chase your dream) and "やっちゃってイイじゃん" (go ahead and just do it). It's a lot of compliments in a row, and that's the point.