It should have messed Marco up pretty badly but
1. It's a cartoon
2. Messing Marco up wasn't the point of the episode, the point was to transfer ownership of the dimensional scissors to Marco and demonstrate the extreme lengths he's willing to go for Star, both of which are going to be extremely relevant to the start of Season 3
It's explained in the in-character livestream which aired after the episode that he basically mentally regressed back to his 14-year-old self shortly after he returned to Earth. He can remember most of the things that happened, but it's only stuck with him like a dream or childhood memories.
I think a more interesting take from the situation at the end of that episode was how Star apparently had no issue with Marco being aged up, and was entirely ready and willing to continue being live-in besties with a 30-year-old man.>>91845960>He ran away from everybody he supposedly cared about, without looking back, as far as he knew and meant for forever, for the sake of his own pride.
That's a pretty harsh take and I'd even say misrepresenting his character.
He didn't want to get the scissors back because he was ashamed of having them, he knew they were important to Star and he felt that he would have been a bad friend not to try to replace them to correct his mistake, especially when Hekapoo made it clear that he had the option to do so.
He was a fourteen-year-old kid basically making a split-second decision on whether or not to tackle this challenge to help his best friend, and Marco is always going to choose to help Star. Even with Hekapoo taunting him, there's no way he took her seriously or really considered how long it might take.
Once he made the decision to hunt down the clones, he couldn't take it back.
And sixteen years later, when his old life was a lifetime ago, making the decision to go back to Earth isn't as simple as just "going home."