Look at autographs of Reggie Jackson, Frank White and George Brett. You don't have to guess who they are.
I read an article about how Harmon Killebrew tried to encourage others to sign their name legible as he did.
I think a lot of factors go into a sloppy sig.
For me, my own signature collapsed into a mess once I hit a job that required frequent and repetitive signing. As hard as I try to sign it the way I want, my brain does not send the signals to my hand.
In some cases, such as celebrities leaving an event, they just scribble as they wall by. Not really a signature but a scribble.
The quality of celebrity signatures has probably been going downhill since the end of the "studio" era in Hollywood. Combine that with the decline of cursive writing and you end up with "scribble" signatures.
You've got to figure current athletes of the big 3 sports today are probably doing 10-20k+ signatures just for the card companies every year. Not counting autograph signings, fan mail/in person, etc. It's hard to blame them for a quick scribble to get it over with. You don't see that with the UD exclusive guys though, where they keep their signatures more limited (Tiger, MJ, Gretzky, etc.)
Many golfers (and I'm sure other sports stars) still have nice signatures.