I first apologize for the lack of details: I read this collection nearly 30 years ago.
I remember only a very few details about it:
- Stories were of various size but at least include a few very short ones
- They were mainly (only?) SF stories
- They were "smart" stories, whose plot was usually the reason why the story was short.
- I remember one story that does exactly match the previous fact (I think it was only 1 or 2 sentences short) : Aliens arrived near earth, saw a sign that said "polluted planet", and decided to fly back home (sounds like that)
- I read it in a French version. The cover might have been light blue (but I am unsure)
This may match a lot of collections of SF short stories, but I hope the only one I remember will help find the one I am looking for!