Interesting topic!
A recent issue by PostNL created by artificial intelligence. The stamps were designed by a computer algorithm based on a collection of 1,500 stamps from various countries in the period 1852-1920.
The design might be state-of-the-art, but these just look weird to me! 
That’s a great topic. I’ll see if I can find a particularly bad one later.
Edit: Have one already.
The USA never disappoints
Another one that comes to mind.
In 2017, for the first time in two centuries, there was a new entrant on the Dutch postal market for consumers called ‘Sandd’. And this was their stamp design:
In 2019, Sandd was acquired by PostNL and ceased to exist.
Polish Post some time ago issued a stamp that suggested that Climate Change was caused by volcanoes - one of denialists’ favourite theories here. The stamp isn’t ugly in itself, but the idea behind it…
Some of these designs are pretty bad. I didn’t like the Suns because some of the images are the exact same image just with a different color. 
Stamp made for the victory at the European Soccer Championship 2020… without the red/white/green ribbon, it could be every country 
I don’t like it, why do not use a photo with the team and the cup? Or the photo of Donnarumma’s block? A missed opportunity…
An interesting question. For many years now, the USPS thrives on producing uninspired postage stamps. In addition to what others have posted, there is no end to stamps designs featuring comics, monsters, etc. Of late, some designs are really in bad taste like “The Day of the Dead,” “Scooby-Doo,” Fruits of all kinds, etc.
It is quite sad compared to what the USPS produced years ago. Not surprising though. Rather than celebrate the culture, the designs selections are the lowest common denominators so as to not offend anyone.
The Dutch AI stamps are just the right kind of weird that I like, would love to see one of those on a card one day!
And the suns on the US stamps look awesome to me, I’m always happy to get one…
I wholeheartedly agree with the criticism of our failed German attempt to communicate the very abstract concept of “forest” on a stamp, I know, it’s really hard to depict…
To contribute something myself:
It’s not the design of the whole stamp itself (which I think is decent), but I discovered that the font used for “80” and “Deutschland” is in fact the infamous Comic Sans - which kind of matches the theme (it’s about the Three Investigators, which is a youth-oriented series to be fair), but the fact that this cursed font is actually used on official German stamps gives me nightmares.
Someone thinks that the Slovak stamp dedicated to Road traffic safety is bad.
Russian post - Hold my beer and watch this:
Honestly, I find a lot of the older USPS stamps to be ugly. The stamps @pjsubway and @meiadeleite posted are definitely two of them, and I’ll add this one:
Ahah, they do look weird… but interesting too, in a way — like seeing something through the “eyes” of a machine! I would use them just so that I could tell the story of them being created by AI.
about several years ago, japan post issued postage stamps of 7 yen or 8 yen of a ugly briefly done ‘graphic design’ that resembles the artwork by Hokusai’s Great Waves. the stamps also had another ugly graphic designed surfer on the sea. like a super terrible parody.
they could have just issued the postage stamps of the original artwork by Hokusai’ Great Waves.
The creepiest and most irksome Sri Lankan stamps of the modern era. One stamp is really racist.
Also, SL Post has a bad habbit of using artworks instead of real photographs for almost all the stamp issues. So some designs are below par in the terms of aesthetics, like this… The photographs would indicate the real beauty of these birds.
This was the stamp for the 100 years of italian state railways, issued in 2005:
I find it ugly and… should it represents railways? 
Take a look to who were other candidates on this article from a good railways website, and judge by yourself if there wasn’t much better to choose among 
http://www.stagniweb.it/fbolli2.htm
That’s interesting because I have often heard complaints from Polish philatelists (regarding our stamps) that photos are used instead of artworks, and that artworks are much better and what should be used on stamps. Personally, I also prefer photos.
And here are some of the really ugly Polish definitives:
This is a fun topic to me. I like many of the stamps that others think are ugly. Like all of these:
I guess it just goes to show that tastes are different.
I generally prefer illustrations to photos, but it all depends on the design. My least favorite stamps are ones with barely any design at all, like the German 3 cent stamp @RalfH showed, or ones that are too busy for such a tiny medium.
