That's a really disingenuous comparison -- the Landmaster is a giant tank that completely obscures Fox/Falco, but the Clown Car is a tiny bowl with Bowser Jr. exposed and in full view.
Not to mention, Bowser Jr. has been portrayed as some kind of mechanical genius ever since Super Mario Galaxy seven years ago. He rides the Clown Car or a big mech in practically every major game he appears in, and he's a feeble joke character on his own (as shown in the original New Super Mario Bros.). His dependence on the Clown Car is a huge part of his character.
The Tron Bonne/SFXTK Pac-Man "pilot" archetype has been missing from Smash Bros., and I'm glad Bowser Jr. is more than just a smaller Bowser. Having a playable mech character really helps diversify the roster.