This slang term would be a useful NOOB, in not having a good U.S. equivalent. Pimply would be the closest, but that sounds kind of weird and maybe too clinical: the kind of thing that Holden Caulfield would say about Ackley. My sense is that in the U.K., spots and spotty can refer to all sorts of blemishes and markings on the skin.
I was interested, therefore, to pick up today’s New York Times and read theater critic Ben Brantley (who has made numerous appearances in these pages) mentioning “YouTube videos of spotty shut-ins making like divas in their bedrooms.”
But I had to go back to 2007 to find another use in the Times: a reference, in an article about skin care, to “half-hour Proactiv infomercials using ordinary people to recount their transformations from reticent loners with spotty skin into pimple-free social butterflies.”
On reflection, I’m inclined to doubt that spotty will catch on over here. To my ears, its teasing overtones (even when used about oneself) clash with the blue-sky self-esteem that our language, at least, insistently promotes.
notoneoffbritishisms.com 
I think you’re spot on.
In the UK it is mildly derogatory too. ‘ a spotty youth’ doesn’t always have acne – but means someone who is adolescent and not to be taken too seriously.. Not meant unkindly, just a mild put-down
Often heard here in the U.S. from a confused newscaster:
“Reports so far are spotty….”
I wouldn’t bet on its acceptance here, either. At least the earlier Times article gave it a context, “spotty skin,” to make obvious its meaning. In my world, definition 2 below (from Google, “define spotty”) is used more commonly, as in “spotty TV reception” or “spotty news coverage”.
spot·ty/ˈspätē/
Adjective:
1) Marked with spots: “a spotty purple flower”.
2) Of uneven quality; patchy: “his spotty record on the environment”.
Spotted dick, anyone?
A pudding, now referred to as “Spotted Richard ” for some reason.
The acne-afflicted Spotty Muldoon was a feature of Peter Cook and Dudley Moore’s UK comedy shows in the 1960s. Here’s his ballad, sung by Peter Cook:
The shows themselves no longer exist since the BBC wiped them off the magnetic tapes they were stored on so that the expensive tapes could be used to save newer programmes.
I thought if something is described as ‘spotty’ it meant ‘hit or miss’. For example, “his rate of picking up a bird in that joint is rather spotty’.
Very late to the party–somehow I’d missed this post–but wanted to note that in British fashion writing, “spotty” means “polka-dotted.” See Boden, which has been inflicting its Britishisms on us Yanks for the last decade or so (“range” for “collection,” “trousers” for “pants,” “jumper” for “sweater,” etc. etc.) http://www.bodenusa.com/en-US/Clearance/Womens-Shoes-Boots/Flat-Shoes/AR502-PPK/Womens-Dusty-Pink-Spot-Spotty-Plimsolls.html
“Spotty Plimsolls,” indeed! This is the US website, by the way.
Spotty is what a dalmatian is. Zitty/greasy/pizza-faced is what a teen is.
I recall it being used quite often as an insult on “The Young Ones,” usually aimed at Rick (“Go to bed, spotty”) and at least once to indicate lack of bathing (“I LIKE my bottom spotty!”).