I love Monster Party! I wrote a whole article on it:
http://www.sydlexia.com/monsterparty.htm
Also, it's important to note that Bandai did not develop the game. The game was developed by Shinsei and Bandai was tasked with toning the game down to conform to Nintendo of America's strict gaming requirements. The original game, which was never released, was much bloodier:
http://dbz.icequake.net/share/afs/pub/emu/nesnew/64.53.95.207/ujap/mp/mp.html
As you can also see, the man-eating plant was drastically altered from its original design, where it looked almost exactly like a Pirahna Plant and was intended as an homage to Little Shop of Horrors. Why the change? No one knows. There's nothing even remotely offensive or violent about the original design. But Bandai was tasked with Americanizing the game, as apparently someone felt it was necessary to alter the design. It's also unclear where the porting was done. Bandai has American offices and produced many crappy toys based on American properties in the 80s and early 90s. As for the Japanese not knowing who Mac Tonite is, especially in 1989, that's unlikely. The Japanese love mascots and they're rather obsessed with American culture.
I do concede that it very well could be an eggplant, but there's no way to know and it doesn't really make any sense, especially given the original design. Turning an evil singing plant into an eggplant would be dumb, whereas turning it into a bizarre homage to an obnoxious singing McDonald's mascot would be less dumb. The fact that results come up when you Google "eggplant" + "monster party" doesn't actually prove anything, except that people think it looks like an eggplant.
The real problem here is that 8-bit sprites lack the detail necessary to give us a definitive answer as to what it's supposed to be. On top of that, many of the sprites in Monster Party are suprising undetailed. The whole point of having giant bosses in games was that it allowed programmers to add additional detail that would have otherwise been impossible given the hardware limitations. Unless someone tracks down one of the Bandai team members who worked on the conversion, we'll never know their intentions. I choose to believe my explanation because it makes more sense.
