Yes, Karna is described as a weak warrior because he has been defeated many times. Several Kshatriya's considered Karna as a weaker warrior. Characters considered karna to be weak and just a braggart:
SECTION CLVII of Drona Parva, (English translation by Kisari Mohan Ganguli)
"Thou boastest much, O Karna, in the presence of the Kuru chief, but thy ...
The story of Arjuna's chariot moving backwards by two steps when Karna's arrows hit and Sri Krishna applauding him, does not find place in Mahabharata.
However, the following incidents did take place, which can be credited to Karna's valour.
1. Karna's arrow misses Arjuna's head due to Sri Krishna's intervention.
The Suta's son then fixed on his bow-...