Automotive circuit to power ATTINY
disclaimer this answer was writing from the point of view that this is to be a simple hobby circuit, placed somewhere where it is always visible and controlled. For something to be permanently installed you need an answer from someone who knows how to make automotive equipment.

You need a fuse on input in case you have a short. The power supplies by car batteries is huge and a short can lead to a fire.

Otherwise this looks good, although that SMPS is quite noisy looking at the datasheet. You might want to add a 100 nF cap on the output, parallel to your 10 uF.

Also that output cap can easily be rated for 6.3V which will make it physically smaller.

Also you might want to look for a diode with a lower voltage drop for less power wasted.