A weak spot for avast in my opinion is the fact that it says it has a "bad reputation" among avast users. Meaning it could be a perfectly good program but just because a few didn't think it suited their needs they rated it poorly.
You are correct about that it can be a harmless add-on. But you can be wrong about the "few", unless it is calculated on percentage.

E.G:
10 people install it, 8 remove it, giving it a bad score of 80%
If avast takes the 10 people and say “Hé look! Only 10 like it…” then indeed it is a few.

I would like to see how exactly avast is calculating the reputation score.
It could tell us a lot more about what is going on.

based on user reviews
I am against this. Almost no-one who is using a computers knows how things are working. That makes user reviews not very trustworthy. Besides that, user who have a problem/bad experience with something post more and faster than user who don't have any problem. So if it should based on user reviews, it would be best to only have a selected group of people who know what they are doing/talking about are allowed to test/review things.

A commonly made mistake (you seem to be making it too),
is that people think the add-on is bad while avast only says it has a bad reputation.
avast doesn’t say it is bad/harmful.
If it would be harmful, the file- and/or webshield should give a alert.

On a side note:
You don’t need the add-on at all to work with Listango.
Just drag the link (as they explain) to your bookmarks and click on it if you want to bookmark a site that you are visiting.