I get where you are coming from, and I see your point, however I disagree.
Matoro's self-pity made a lot of sense, he just left his home, went through Karzahnii, and has now become a Toa. All while the Toa Nuva AND Mata Nui were dying. They are facing a threat that bested the Nuva, the most powerful of all Toa...and he talked and took hikes.
The rest of the Inika had skill-sets that would considered useful, Jaller was the captain of the guard and led many an attack on the Rahi, Kongu commanded the Gukko force, which played a large role in the fight against the Rahi as well, Nuparu was a engineer, who designed the machines that fought off the Bohrok. Hewkii was a sports champion, proving his strength and cunning. Hahli was ALSO a Kohlii champion, and depending on just how canon MNOG 2 is..she also did whatever she did in there.
Matoro though, he only translated for Nuju, which is a big task, but pretty useless to someone who has to be a warrior. His other accomplishments only add to his depressed state. The Kohli, which he lost. the hiking, which is nothing special, he wandered in the snow, big whoop. He once guarded Kopaka's Nuva symbol...and was beaten after being made too heavy to move.
Matoro likely always had a lingering sense of depression or loneliness, which was only made worse when he was forced to become a Toa, then was haunted by the Ignika, and later forced to die for the fate of the universe.
Now as for Kongu, as poorly as it was explained, worked well. considering him and Jaller were just a bit too similar as characters. (With both being seasoned military leaders and all) And one can assume that Kongu is somewhat like this when he is not being super stern and stressed out. His character in 06 was likely because of who he was with, his friends.
Anyway, just my two cents.