Not necessarily so:
Classic Examples:
original - Bob James "Mardi Gras"
song that sampled it - Run DMC "Peter Piper"
original - Little Feat "Fool Yourself"
song that sampled it - A Tribe Called Quest "Bonita Applebum"
More Recent example:
original - The Cyrkle "Neon"
song that sampled it - A Tribe Called Quest "Get a hold"
Unless you like cheesy elevator music, country rock, and music by  
the Mamas and Papas, the original songs mentioned are all wack. It's  
true though that a lot of the original songs sampled in hip-hop  
tunes also happen to be some of the best music ever made. But more  
often, like KRS ONE (in a cover of a popular Beatles tune) - "...We  
take the wackest song... and make it badder" - that's hip-hop.
One the same topic, I found a web site the other day - original  
songs and hip-hop tunes that sampled it. Very extensive (a 250 K  
text file) and covers a lot of ish, but even the authours mention  
it's still not complete yet - interesting site if you're into who  
sampled what and info like that.
http://sparrow.accs.howard.edu/staff/marc/rap.txt
shanti,
ashwin
ashwin@visgen.com