Is "patzer" a bad word?

Is it?

Depends. It's a german word in the origin.

patzen - make a mistake

As failing is human it depends from the usage.

Usually it is used as 'weak chess player'. Weak relatively to who's speaking (in which case it is condescending), or objectively weak (in which case it is just a truth).

idk

Patzer is also a very over-used word by people in a condescending way towards people who have a lower online 'rating,' even if their opinions are valid. Really though, anybody who throws around the word "patzer" like it's some original 'insult' is an idiot, and compared to people who are better than them at chess, a patzer as well.