i just want to know if men appreciate these types of things?

Yes, a lot of people of both genders appreciate custom gifts.

if you’d feel flattered by a mixtape from a potential s/o, or think it’s a little weird?

Both. The thought is nice but realistically this guy won't have a tape player unless he's specifically into that, or you know he has one in his car.

would you rather get a mixtape with songs you have shared moments to, or of songs you’ve never heard that remind that person of you?

Shared moments with. There's no guarantee he'll like a song he's never heard.

the general consensus on mixtapes?

Obsolete, but cool mementos of the past.