I think giving Rant a friend in the underground would fix him, but it'd also make him worse in a completely different way. Like he'd be the most co-dependent, attached at the hip little freak you'd ever meet. He's still dramatic and hypersexual, he's just not as rude about it and doesn't emulate Mutt's behavior when he starts breaking hearts.
He'd also probably grow a small resentment for his brother in this timeline, he still thinks he's the coolest and wishes he was more like him- but with a bit more self confidence from a friend, that same someone else paying attention to him, and them dragging him out of his house, it's a small feeling of abandonment that festers as the years go on because this friend is physically there for him more than his so called brother ever has been. And yet Mutt continues to have the AUDACITY to hate his friend. Because Mutt would, legit, HATE them because he's ✨Overprotective✨ but only when it comes to lasting figures in their lives. If it's a random person he's taking home for a one night stand thats just fine but someone that stays? Mutt himself is uncomfortable with the concept because he's spent so long without anyone choosing to stay for him that he reacts negatively to genuine care. And because he sees Rant's social life as an extension of his own, because in his mind all they have is each other, Rant having a genuine close friend Also triggers that discomfort and makes him feel like Rant's being stolen away.
Anyways i've been reading those fics where reader falls into the underground and grows up alongside the skeletons and gets thrown into another universe with them and was wondering how it'd be for my boys and yeah... it'd be interesting.
Over time Mutt would soften up, the more he visits and sees the friend genuinely cares about Rant and that they Aren't Leaving no matter what, when they cement themselves as a constant in his mind he'll start being more teasing with his jabs and less hostile until they have a more friendly, affectionately insulting relationship. He'd gain a respect for them when he sees them helping Rant in a time of need when Mutt can't be there due to his job, and sees that they care as deeply for hir little brother as he does. This would also make the friend the first person Mutt's never seriously flirted with, as Rant would Actually Kill Him (Not really but it'd be a HUGE fight). He'd still mess around and jokingly flirt but it would never mean anything, because Rant would actually get jealous with this specific person. He's fine with sharing, just not this one. This is his person.