Is there a way to change a LUT effects opacity? Seems simple on Premiere but can't seem to find it anywhere in kdenlive.

A LUT is simply a remapping of the RGB space, so changing opacity is not well defined.

You can try to use two copies of your clip stacked together with an affine transition, apply the LUT on the topmost one and fiddle with the opacity of the topmost clip, but I'm not sure it'll give you what you expect.