I honestly think he meant it so that Mahoraga would give his all as he is Sukuna’s now which means he’s fueled by Sukuna’s CE instead of Megumi’s (or I guess that’s what I think).

In my understanding, いい and 良い(よい) have the same meaning of “good” or “great”, just that the former is more casual and more commonly used than the other and there are also differences in how it’s used in a grammatical sentence (like you have to use yoi to say よかった, not いかった). But as I tried to dig more in the differences, I guess it’s that 良い are naturally used only in positive contexts, but いい is more widely used as it can naturally be in both positive and negative contexts (like “good riddance”, maybe, lol). on the other hand, 良い(yoi) are used in some phrases like “have a good year” or “have a good weekend”.

But in Sukuna’s with Megumi and now Mahoraga. I think he both meant it as a praise (“great”), so it’s the same meaning, yet you can see he was more excited in seeing Megumi’s development, looking from his face and how he’s repeating the “great”. He’s more casual with it in my opinion, like he’s surprised and he’s proud. On the other hand with Mahoraga, seeing his expression and how he’s being kinda formal, it looks to me that it’s a praise for something that is already expected. Granted he’s also in the middle of a fight so he needs to stay cool and calm as well lmao but it’s also interesting the way Sukuna covered his mouth while doing that.

It’s just my interpretation anyway. I’m not and far from being a native so I can be wrong.