The only issue I have with how Jankom is portrayed is that he's so much shorter than the remainder of the cast. Tellarites are not under four feet, which is what seems to be suggested.
Of course, despite having a deep voice, it's possible he's actually meant to be a child, not a teenager like Gwyn and Dal. Maybe Tellarites have deep voices from the beginning, and they only have a growth spurt in adolescence where they shoot up two feet.
It's also possible he's just short.