looks to be a low cost/cheap bit ....
That bit would probably not give great results in plain hardwood, let alone Baltic Birch plywood. Plywood, even with the best of bits won't be easy to cut without splintering. because the grain keeps changing 90 degrees to itself. On hardwood it at least runs one direction ... along the length which makes splintering minimal.
I don't think a zero clearance plate would make that much difference, but it won't hurt to experiment. Also make progressively deeper cuts. Also, you may have to "climb cut" to avoid splintering, and that's a bit dangerous. I would just make a 45 degree miter on the table saw and use a spline for strength and positioning.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)