It should work, I've seen it done before. Just make sure the capture zone trigger lines up with the sloped platform (I recommend the vertex tool for this), and you should be fine. I don't see it having any major gameplay effects that would make comp players look the other way.
Just did it in my 6v6 map cp_banana and the comp playtesters didn't mind one bit, it wasn't even mentioned. With the amount of other stuff they nitpicked, i think you're fine with sloped CP's.
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