It's 95% the camera, not the film, although there are ways that the film can contribute. For example, you can load a roll of film backwards to get redscale. Or you can shoot expired film, or film that has been "ruined" in some way (there was a thread recently about putting film through the dishwasher, drying it out, and then shooting it. Which you wouldn't want to do with 120 film!)
I doubt seriously you'd get typical lomo effects from the Mamiya without faking those effects in post.
When I first wanted to experiment with lomography, I bought a Zorki 6 rangefinder and then a Lubitel TLR. The Zorki definitely had some light leaks at first, but I decided I didn't like them so I fixed the seals. Then I realized that the lens was just too good to waste on lomo shots. The Lubitel too, crappy little plastic thing that it is, proved itself too good for that intentionally-flawed Lomo look.
A Holga 120 is about $35, and a 35mm Holga is probably similarly priced. Or get a Konstruktor kit (35mm) if you want to have the fun of putting the camera together yourself (it is actually pretty fun. Except for the B10 spring. That one will drive you crazy.)