I have the exact same filter as you, so maybe I can explain. The filter's intake is what determines how well the filter works/how powerful it is, since the motor is pulling water up the tube at a steady rate. Some models are adjustable, or you can put a piece of dense foam over the intake tube which will slow it down, but that's not imperative unless your betta is getting sucked up against it. (Old, sick, or young fish may need this covered, but a full-grown betta is usually fine.)
The output area is what you want to baffle, because the flow of water pouring out is going to create a strong current that goes all the way to the bottom of the tank. With a baffle, you reduce this flow by prematurely breaking the current. As far as I know this doesn't have negative side effects with the filter function (Since again, the intake tube determines how fast the water gets sucked in). There might be less oxygen in the water if you decrease surface movement, but bettas don't really breathe from the water so it's good.
You can either take a piece of foam and put it across the entire opening, holding it in place with a rubber band, which will break up the flow and even give you some extra area for good bacteria to grow. Or you can make a water bottle baffle, which is explained wonderfully and with pictures in this thread
. Basically you cut the top and bottom off a water bottle, then make a vertical cut up the side so it's a curled tube of plastic. Hold it sideways and tape one of the cut ends to the top of the filter, letting the other curl under
the filter's output. The water will hit the water bottle and instead of rocketing down to the bottom of the tank, it will flow much more gently out the sides.