I had issues using the isometric tilemaps feature in Unity. Whenever I try to place tiles, they don't seem to line up and create this jagged effect. I've tried messing with the anchor points and am a complete beginner at this.
Judging by your sceenshot, it looks like the top diamond of your sprite is about 104 pixels tall by 181 pixels wide - though the scene view might be resampling it at a different resolution than your source asset, so be sure to measure in your original texture to get the exact measurements.
That's about 57.5% as tall as it is wide. So in the Cell Size field of your Grid component, you'll want the height to be about 0.575 to a width of 1.0, instead of the 0.5 you have currently.
Again, this is a bit of a crude calculation based on the screenshot, so massage the numbers until you find a fit that looks right.