A scowl is a sort of a thing you may find stuck to something else. One place you're likely to encounter it is on overloaded dumpsters in back alleys. There are already lots of other things stuck to them, and it's not really surprising to find one more stuck to them, and so we find the back-alley garbage scowl. If you find this on one of your dumpsters you should take it seriously, because it means your dumpster is unhappy. The next stage, if you continue to overload it, is when the dumpster becomes revolting, and who knows where that might lead.
There are other kinds of garbage scowls, too. Famously, the Starship Enterprise was also said to be a garbage scowl. The person saying it was a Klingon, though, so he may have been wrong.
The implication is that, in the world of the future, we'll need to send our garbage from one planet to another in order to find someplace to dump it. Shades of the Break of Dawn!
In fact we can be pretty sure Korax (the Klingon) wasn't right about the Enterprise being a garbage scowl. For we saw lots of views of the Enterprise and it didn't look anything like what we see here, which is a typical garbage scowl on an intersystem run attempting to find a landfill that isn't filled.