There's nothing bad about wanting to delete or hide songs to prune your playlist a bit.
This will depend somewhat on what console you're on. Assuming you're on the XBox 360, if you're on Title Update 4 (necessary if you play customs), unfortunately there is no easy way to hide or delete songs that were downloaded in a pack, which includes the export packs from previous games.
If you're on Title Update 5, the most recent update, you can rate those songs 1 lighter, and then go into your settings and enable the option to hide 1-lighter-rated songs. The songs will still be on your hard drive, but otherwise they're effectively shunned from the game, at least until you uncheck that setting again.
If you're not on the 360, let us know what console you're on and hopefully someone more familiar with that console will chime in.