First said, I believe, by an interior designer, but I can't remember who. Please help! Thanks.

Most Sources attribute it as a paraphrase to Duke Ellington:

if it looks good, it is good (to paraphrase Duke Ellington).
http://home.nc.rr.com/mikemurphy/information/index.htm?http://home.nc.rr.com/mikemurphy/information/worth.htm
http://www.clarkgallery.com/shows/2001/2001-01-30/show.html
http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=003EYX

B.B. King once said, “If it sounds good, it is good.”
http://www.harveygallery.com/geometric_-_jazz.htm

To paraphrase Miles Davis, if it looks good, it is good.
http://www.pcphotomag.com/content/pastissues/1999/nov/printers.html

To paraphrase Peter Schickele (quoting the Duke), "If it looks good, it is good.”
http://www.bantjes.com/marian/archives/000154.html



"If It Looks Better, It Is Better.
www.paintstore.com/pwc/0699/cutting.html

I asked Jesus, "How much do you love me?".
Jesus answered, "This much"
as He stretched His arms and died on the cross for me.
-- Unknown, (Author / Orator unknown or unconfirmed) Full story here

Couple more:

"If it looks good, it will sound good."
~ old maxim
http://www.stereophile.com/printarchives.cgi?606

"The most important thing to keep in mind is that it's your home, and your rooms should reflect your style and personality," said Margi Daniels, ASID, design consultant for La-Z-Boy® Chair Co. "If it looks good to you, it probably will look good to everyone else."
http://www.recordernews.com/bridal/stories/create/create.html

The quote I saw was from a designer, sometime from 1900-20. She mixed styles, which was not common then in 'high-end' homes.
Thanks for the help.