If the angle is too steep then you'd need to pack out the front of the neck joint. It's more normal that you need to increase the angle, and a small hard shim right at the rear of the pocket will do this. But any kind of shim at the front end of the pocket will leave a visible gap. Also, you only really get the neck making contact with the neck pocket in two places, instead of all over.

Any shim needs to be hard - which is why epoxy paint is suggested, but you can also use hard thin plastic or metal.

But I'd be more tempted to sand the bottom of the neck to get the angle right (it shouldn't take too much time to refinish it). I'd certainly try a shim at the front of the pocket (headstock end) first to see if it solves the issue and also to get an idea of how much needs to be removed (probably not a lot). You then have to try and sand the bottom of the heel at an angle and without taking too much depth off. That's the hard bit.

You could probably draw the angle on the heel, then put the neck in the jaws of a workmate or something similar so that only the bit to be sanded off poked out over the top, and then sand it down using a sanding block or sandpaper stuck to a piece of wood.

BUT - before doing any of that, first check that the neck is fully screwed down and there aren't any wood splinters around the screw holes holding the neck off the pocket. Also check that the end of the neck/pocket is shaped correctly so that the neck is fitting properly in the pocket and not being held high at the corners (which will increase the neck angle).

If the screws are being screwed into the body first before screwing into the neck, then I would enlarge the hole size with a drill (running the drill in reverse is the best way to do it slowly and without risk of splitting the wood) so that the screws fit through the body holes without biting into the wood, which allows them to pull the neck down properly. If the screws are biting through the body and the neck, then there's no guarantee that the screw is puling the neck down as hard as it should. This was the recent cause of a bass neck having a high angle - so it can happen.

Only if you do all that and then still have an issue, would I think about either shimming or reshaping the neck heel.