Harley Quinn is actually from the Looney Tunes universe.

For one, she's not an original DC creation. She was created by the WB animation studios.

On Batman: The Animated Series, her movements and actions are considerably more traditionally cartoony than all of the other characters.

The writers and directors of TAS even admit in the commentary that the scores for the episodes that focus on her have a lot more cartoon sound effects than ones that don't.

Her weapons of choice in the series... a giant mallet and a spring loaded boxing glove look more like something Wile E Coyote would use. Sure, other characters, such as The Joker have themed weapons too, but for the most part, the style is purely cosmetic. Harley's weapons also tend to make more of a cartoon "pop" than those of most of the other characters.

And does that thick, nasally Brooklyn accent remind you of a popular Looney Tunes charcter, doc?

I mean, she's basically a gender flipped version of the Joker, right? Didn't that particular Looney Tunes character also have a penchant for cross dressing?

Perhaps most importantly... Harley really, REALLY loves her Looney Tunes:

https://www.youtube.com/watch?v=qbDeoXR_9gc

edited 28th May '15 12:20:41 PM by Devitron5000

If so, then she's probably related to Judge Doom. They're both more disturbing than silly.

She's definitely become more disturbing since she became a canonized DC character, but the connection is more apparent in the animated series. On one of the commentaries, one of the producers talks about working on Tiny Toons when he was called on to do Batman, which I think sort of explains the connection.