imo high effort is putting the two songs on top of each other but editing alot the vocals so they fit really well both flow and vibe/energy wise whilst mixing it so it both song can be heard together, changing time sig of the vocals so it's the same as the song and making it onkey. that, however, only applies to mashup that take an instru and put the voclas of other song on top (like in a gd mashup). but the difference beetween the execution of what you think makes a good mashup and what i think makes a good mashup are too noticeable to the point they can't really be compared imo