No, no, no. A kumara is not what you're thinking. This is a kumara. OK, maybe it looks a little like what you were thinking but this explains the phrase. It means that something didn't go quite as expected, with disappointing consequences. For example, Pox's internet fails just before killing the boss in Molten Core. It took him a couple of hours of dungeon crawling to get to that point and now he has missed out on the drop he went in there to get. It would be entirely appropriate for him to declare in guildchat: "MC sucked the big kumara!"
Kumara was traditionally cooked in a hāngi, which is a kind of barbecue: a pit of hot stones in the ground, layered with food and then covered to cook. This is a hāngi.
Sucking the big kumara is not to be confused with a similar phrase: suck it and see. This is used when you come across something new that you don't fully understand yet. One would "suck it and see" upon entering a World of Warcraft dungeon for the first time, for example. It means trying something new to see whether you like it. Try this experiment: cut a lemon in half and stand with it in front of a brass band while they're playing. Now suck it and see what happens!