Mondo quickly established himself as my favourite Trigger Happy Havoc character when I was first working my way through the story, and, although Makoto ended up overtaking him, I still have a soft spot for Mondo. There are a few character types I've always had a weakness for, and, in particular, I'll probably like any character who can be described as an angry disaster.
Mondo's particular blend of angry and basically well-meaning really works for me! He's a scary guy, but his heart's in the right place, and he can be oddly endearing sometimes. I love the friendship he ends up forming with Taka. I love how he defends Chihiro against Byakuya, and how he blushes when Chihiro thanks him. It took me a while to remember the characters' names, so for a while Mondo was just 'Strangely Adorable Gang Leader' in my head.
After punching him out at the beginning of the first chapter, I like to think Mondo was the one who carried Naegi to his room. Mondo and Sakura are the tallest and strongest of the group; one of them would have been Naegi's mysterious transporter, surely. Immediately after knocking Naegi out, I think Mondo probably came to his senses and went, 'Fuck, guess I'd better get this kid somewhere safe.' It'd certainly fit with everything we end up learning of his character.
Mondo isn't a bad guy, or at least he doesn't want to be a bad guy. He's a compassionate person! But he's also impulsive and violent, meaning he ends up hurting people and then desperately trying to make up for the harm he's done in whatever way he can. It's a fascinatingly tragic combination of traits, and, in the setting of Danganronpa, its consequences are devastating.