I think All the other Trolls have Rounded Ears, just like Biggie or Guy Diamond, see? 

- And Branch Also HAD Rounded Ears as a Kid when he was still singing & Happy ( before losing his true colors ) 

So What Happened to Them? - Why did his Ears change their shape? 

( My Theory is: To underline His Unhappiness when he was Grey & to make him look 'as different from the Happy Trolls' as possible: His pointy ears made him look Sadder when he was Grey - but More Masculine than other male Trolls when he got Happy again - so Pointy Ears just Fit the Adult Branch - & I think that everything that I could say about it was Already Said :) )

< As you can see: I'm a good observer - & pay attention even to details like That ⤴😉👍 >