We have to remember that Bhishma did not talk about Karna alone, while catergorising various warriors on both sides of the war. He even stated in an unbiased manner that Drona’s son Ashwatthama is also not even a Ratha or Maharatha.
We have to remember that Both Bhishma and Drona were disciples of Parashurama, who was the teacher of Karna also.
Ashwatthama did not protest at Bhishma’s assessment. And, even Drona agrees to his evaluation.
क्रोधस्तेजश्च तपसा सम्भृतोऽऽश्रमवासिना । द्रोणेनानुगृहीतश्च दिव्यैरस्त्रैरुदारधीः ॥
दोषस्त्वस्य महानेको येनैष भरतर्षभ । न मे रथो नातिरथो मतः पार्थिवसत्तम ॥
जीवितं प्रियमत्यर्थमायुष्कामः सदा द्विजः । न ह्यस्य सदृशः कश्चिदुभयोः सेनयोरपि ॥
Through his austerities in the hermitage, he has heightened his rage and strength. Gifted with great intelligence, Drona has blessed him with celestial arms. However, there is a significant flaw in him, due to which I do not see him as a Ratha or Atiratha (not a true warrior). This esteemed man has a deep love for life, and life means everything to him.
भीष्म उवाच॥
अचलो वृषकश्चैव भ्रातरौ सहितावुभौ । रथौ तव दुराधर्षौ शत्रून्विध्वंसयिष्यतः ॥
सखा ते दयितो नित्यं य एष रणकर्कशः । प्रोत्साहयति राजंस्त्वां विग्रहे पाण्डवैः सह ॥
एष नैव रथः पूर्णो नाप्येवातिरथो नृप । अभिशापाच्च रामस्य ब्राह्मणस्य च भाषणात् ॥
करणानां वियोगाच्च तेन मेऽर्धरथो मतः । नैष फल्गुनमासाद्य पुनर्जीवन्विमोक्ष्यते ॥
सञ्जय उवाच॥
ततोऽब्रवीन्महाबाहुर्द्रोणः शस्त्रभृतां वरः । एवमेतद्यथात्थ त्वं न मिथ्यास्तीति किञ्चन ॥
रणे रणेऽतिमानी च विमुखश्चैव दृश्यते । घृणी कर्णः प्रमादी च तेन मेऽर्धरथो मतः ॥
Bhishma said,
Both Achala and Vrisha are great charioteers. Unbeatable in combat, they will defeat your enemies.
As for your ever-loyal friend, this one who constantly brags about his battle skills, this one who always pushes you, O king, to fight the Pandavas, this arrogant fool, Karna, your advisor and companion, this conceited man who lacks wisdom, Karna is neither a great charioteer nor a formidable one. Due to the curse of Rama, his mentor in war, and another curse from a Brahmana, as well as his loss of battle gear, I believe he is merely half a charioteer. When he confronts Phalguni in battle, he is unlikely to survive!'
Hearing this, Drona, the most skilled warrior, responded, 'What you say is true! He boasts before every battle, yet he always withdraws from every fight. Compassionate yet incompetent, that’s why, in my opinion, the son of Radha, is only half a charioteer!”
