What is an adjective that describes something very visually crowded or busy? Cacophonous is perfect, but it describes sound.
Depending on your specific situation, I think chaos, confusion, turmoil, and tumult all have the chance to do nicely. However, I also like the potential for metaphor here; if you can use it in this context, why not just use cacophony or visual cacophony? We even use loud in English to describe brightly colored things; sound can be used to express a visual experience so intense that it seems to bleed into the other senses.
Perhaps one could describe the thing as "cluttered":
1.Scattered with a disorderly mixture of objects; littered
For example, "The table top was cluttered with an endless amount of items"
There also the other examples of croweded, messy, littered, dishevel, scrambled(like scrambled egg), and my personal favourite, topsy-turvy
Yiddish has the wonderful word ungapatchka, which means overdone, garish, distastefully ornate, or over-the-top; I think it's exactly what you're looking for. Too bad it doesn't have the currency in English that some other Yiddish words do.
"Garish" is not a bad English word for this, come to think of it!
Garish is the closest I can think of.
Cacography describes bad handwriting or scribbling. You could help reenforce cacovisual as a new word. It was coined elsewhere but only has two English references on the web according to Google.
You might describe it as assaulting (one's senses), or a visual train-wreck.
Some related words and phrases are groteque and eye sore.
Down south, we say "gaudy", and although this tends to be a term of disparagement used against presumptuous decor and attire it is often extended to other situations, as well.
"The colors they used in that mall architecture were just gaudy; i can't look at the place for longer than two minutes, and it give me a headache."
David Brin appears to have coined the word cacophote in his book Sundiver, to refer to a visual cacophony.
See this excerpt from the book. I instantly understood the meaning upon reading it, but the word does not appear in the OED, and a google search turns up only this passage.