My sister lives in Columbus, GA and at the entrance to her neighborhood is one of those "soil islands" that nobody ever takes care of. I know that years ago my BIL would weed-eat it and prepare it for planting. Then my sister would plant different flowers there. Neighbors appeared to appreciate it. Whether "right" or "wrong" to do so, I believe the net outcome was positive. I'm not sure she's still maintaining it or not...she's older now.
Who really owns that little spot of soil? They apparently weren't taking care of it and apparently didn't mind someone else beautifying it. Certainly you wouldn't want to plant something exotic that you bought off of eBay and came from China, but annuals or even perennials that are/have been domesticated and aren't invasive should be ok. Sure the "powers that be" might be encouraged not to do anything if someone else is taking care of and beautifying a spot but rest assured that doing nothing isn't going to get them off their lard azzes, either. Sure, you could go to several city council meetings, moan and groan, and eventually a city crew might come out and un-enthused workers could plant "something" and after a while let it go back to weeds or they could come out RoundUp the soil island or cement over it. Or, someone who cares could do some responsible guerrilla planting and put a little beauty in an otherwise sad and forgotten corner. I mean, only people of little character and appreciation for nature and those experiencing past frontal-lobotomies would not understand the positives from these kinds of actions. A flower lifts most anybody's spirit.