At the risk of DJ-splaining, a medley is a varied mixture of people or things; a miscellany, like a bag of frozen mixed vegetables or a 4 AM tavern. (Remember those?)
To mash up is to mix or combine two or more different elements, like a bag of miscellaneous meat parts and a bag of frozen Asian style vegetables. In this case, by definition, the following qualifies as both a medley and a mash up.
I never realized how much Rush and Diana Ross & The Supremes had in common until I forced them to coexist on the same plane of music. They are both power trios. They are from Canada. They are Black. They have many fans. They are old. The only apparent difference is that Geddy Lee has a higher singing voice than Diana Ross & The Supremes.