Yes, Disguise Self is enough to appear like a Death Knight.
You make yourself look different until the spell ends or until you use your action to dismiss it. You can seem 1 foot shorter or taller and can appear thin, fat, or in between. You can't change your body type, so you must adopt a form that has the same basic arrangement of limbs. Otherwise, the extent of the illusion is up to you.
I don't think that you can appear like skeleton, because it would not qualify as appearing thin, fat, or in between.
However, Death Knights wear plate armor, which covers the entire body so you don't have to appear like a skeleton, you just need to appear like you're wearing plate armor with glowing eyes.
Disguise Self places no constraints on those two things, so you can do them.