Even on 100% acrylic that doesn't block, a wash and block will even out the stitches and make the knitting look better. In this case, it may not block as extensively as something that's 100% natural fibre, but 60% is significant. I would definitely wet-block, I don't recall anything about mohair being a particularly fragile fibre...
Actually, be careful about steam on acrylic, some people like the effect and say it softens and relaxes the fabric, but others describe this as killing the item, making it go limp, and hate the effect. If you want to try this, I suggest knitting up a little square in the same pattern, same size needles, and steam that. This way, if you hate the effect, you haven't ruined a huge poncho. Even with only 40% acrylic, it might go limp, and this effect is irreversible.