Because the Coral Reef is located in one of the parks, I always assumed it was fine to wear whatever you've been walking in all day. It's still a lovely restaurant, though, so if you want to dress up, I think it's a nice touch.
I haven't eaten at Boma, but I have eaten at Jiko's. Now, at Jiko's when I went to sit down after dragging my surplus of bags and souvenirs in with me, I felt horrid.

Next time I go, I think I'll stop off at the hotel first and dress a little less park-savy and a little more fine dining. That said, the staff never batted an eyelash or made any sort of rude remark about our clothing. They treated us well, which felt great. So, if you ever feel nervous about your attire at any WDW restaurant, remember where you are, and that the staff is absolutely awesome, they probably won't care!
Since Boma was right next door, I got to take a peek and it seemed like the total polar opposite of Jiko's. (Though it smelled fantastic...!) I saw heavy crowds, lots of families and everyone was pretty much casual. It's safe to say that if you go to Boma, you'll be fine in your park clothes!
