Meelo's eyebrows are actually typical of what the Heian/ancient Chinese aristocracy considered beautiful.
This was the height of fashion - aristocracy often plucked their eyebrows entirely and painted in shapes like that - you can kind of tell from the Yoshitoshi print. Eyebrows were often painted much higher than where they originally grew it - I suppose it made the eyes look smaller, which was also a favored trait. Women were also expected to black out their teeth. I have no idea why any of this was considered attractive, but there you have it.
As for the others, their relatively thicker eyebrows might be a subtle nod to their lower standing in society, or the character designers may have just thought 'Man, a short barrel chested guy like Bolin would look SILLY with thin eyebrows, lets thicken them up' and similarly for Mako. It might also be a masculinity thing - The general consensus seems to be that Bolin is manlier than Mako [teehee], so his eyebrows are thicker.
There may ALSO be an aspect of what's considered attractive NOW. In today's society, the thinner, more shapely eyebrows are considered attractive, (hence nearly every person on 'Jersey Shore' waxing their eyebrows into oblivion.... not that they're attractive...) thus the main character's love interest has better shaped eyebrows than his brother, whom she never really saw as a potential mate. If you're referring to that odd little curve they do at the arch, that's just a typical exaggeration you see in animation, like gigantic eyes.
There's a lot of wiggle room when it comes to mako+bolin.
Tenzin's are just bushy.