>Do I need a amp/dac

Much like with headphones, it depends on the IEM and the random headphone jack in question. If you want to be safe, get something that is a known quantity.

Aside from noise, which is obvious enough, IEMs are usually low-impedance so you're most likely to run into trouble if your source has a high output impedance relative to the impedance of the IEM, which is unfortunately not a spec you can usually find for headphone jacks that aren't on dedicated headphone amps. Since the Dioko is a planar single-driver IEM, I assume its impedance doesn't vary with frequency, so this is less of an issue since this will only affect volume, but many IEMs have goofy impedance curves and inadequate amplification will significantly affect the frequency response.