For the boss races in Slime Speedway, you are normally required to face the boss as someone from the same series (IE: Squidward is faced as SpongeBob, April is faced as Leonardo, Gerald is faced as Arnold, and Powdered Toast Man is faced as Ren). The only two bosses that don't follow this pattern are Oblina and Jenny, as they are the sole representatives of their respective series (Aaahh! Real Monsters and My Life As A Teenage Robot respectively). However, the characters you play as in the races against them still make quite a bit of sense.
For Oblina, you race against her as Chuckie. Beyond the Actor Allusion (both were originally voiced by the late Christine Cavanaugh) and creator that the duo share (not to mention, Oblina actually did appear in an episode of the original Rugrats), there is also the fact that Chuckie is a Lovable Coward while Oblina is a monster who delights in scaring others. Chuckie is most likely trying to face his fears by beating Oblina.
For Jenny, you race against her as Jimmy Neutron. Jimmy has dealt with his fair share of robots in the past (the Willie Loman 3000, Nanobots and Copbot all come to mind), so dealing with Jenny wouldn't be anything strange for him.
