The leaves on Tomy-T's head are tomato leaves. This refers to his Japanese name "Tomato" and his English name ("Tomy-T" and "tomato"; also the "T" standing for the "to" in "tomato"). He is never seen without his red sweater, which also makes him look more like a tomato.
Tomy-T has the unique trait of turning a furious shade of red. At first, it looks as if he does this when angry or frustrated, but then in Marron's Autumn Festival he does this when the Ham-Hams say they will help them with the harvest because that's what friends do. So it may just be when he is under intense emotion.
He is apparently a very bad singer, according to Marron.