Any fish can jump even if they're not known as jumpers. Just because they jump after months of not jumping doesn't necessarily mean somethings wrong with your water. I know sometimes I have to turn on nearby lights at night and my fish freak out and I wouldn't be surprised if they accidentally jumped.
I'd say putting on a top is great for all aquariums but yes it does look uglier. I personally made a window frame to go on top of the tank so it just protrudes out a bit on the edges but otherwise is rimless (you could add a plastic mesh on it as well). I think if you take away the edges they will not jump as much... they still could but it's a lot better than not having something in the way.