Taking some time out to write a top ten, to explain this one it captures cartoon theme songs that either use public domain music, sounds way too similar to another piece of music or out right steals a song and adjusts this for their own tastes.
Honorable Mention: Tom and Jerry
It's entire soundtrack is Metro Goldwyn Mayer's entire music catalogue, for an example you can sometimes hear Over the Rainbow from Wizard of Oz. Left off because I'm only covering opening themes and Tom and Jerry's theme is unique to them and a few other MGM cartoons.
10. Smurfs
Quite low as it's unclear which came first, but it sounds similar to the song Centerfold by J. Geils Band
9. Dennis the Menace (US version)
I say US version as Britain has it's own Dennis the Menace. Weirdly they did have a servicable theme song but went public domain for future series.
8. Wacky World of Tex Avery
Public Domain again, added bonus the actual opening is disgusting.
7. Berenstain Bears
Probably the most shameless use of public domain music.
6. Inspector Gadget
It's Hall of the Mountain King, another Public Domain track, I'd compare them.
5. Scooby Doo Get A Clue
They ripped off the Offspring, it sounds too similar to One Fine Day.
4. X-Men
As awesome as this theme song is, it's Whitney Houston's I'm your Baby Tonight.
3. MASK
Childhood not ruined yet?
Credit to UncannyX, my inspiration for this list.
2. MadBalls
This show failed to go beyond two episodes mainly because it stole Jerry Lee Lewis' Great Balls of Fire, they didn't even try to hide it.
1. Swamp Thing
This is the most embarrassing one, not only does it destroy a good rock ballad, in this case Wild Thing by The Troggs but the lyrics they chose are some of the worst I've ever heard, an easy candidate for worst cartoon theme song.
