In serials, it is always shown that Karna called Draupadi a whore (Veshya) when dice thing happened and Pandavas lost everything. If yes, why they say he knew Dharma?
Also, if yes, was he right to say so?
Yes, he did. The exact word Karṇa uses to refer to Draupadī is bandhakī (बन्धकी):
02,061.035a eko bhartā striyā devair vihitaḥ kurunandana
02,061.035c iyaṃ tv anekavaśagā bandhakīti viniścitā
...which translates to:
bandhakī f. ("connected", scilicet with many men), an unchaste woman, harlot, courtesan
Bibek Debroy uses 'courtesan' in his translation of that verse.
Sabhā Parva / Dyūta Parva
Chapter 286 (61)
When the noise died down, Radheya, who was almost senseless with anger, gripped his lustrous arms and uttered these words, “I have witnessed many distortions in Vikarna. Like fire destroys the block from which it has been kindled, his destruction will come from the fire he has created. Though urged by Krishna, those who are assembled here have not uttered a word. I consider that Drupada’s daughter has been won in accordance with dharma, and so do they. O son of Dhritarashtra! Out of childishness, you alone are being torn to bits. Though but a child, you speak in this sabha what should be spoken by elders. O Duryodhana’s younger brother! You do not know the reality of what dharma is. Like one with limited intelligence, you proclaim that Krishna has not been won, when she has been won. O Dhritarashtra’s son! How can you think that Krishna has not been won? In this sabha, the eldest Pandava staked everything he possessed. O bull among the Bharata lineage! Droupadi is included in all his possessions. When Krishna has been won in accordance with dharma, how can you think she has not been won? Droupadi was mentioned in the speech and the Pandava approved. According to what reason do you then think that she has not been won? If you think that bringing her into the sabha when she is clad in only a single garment is against dharma, listen to the words I have to say in response. O descendant of the Kuru lineage! It has been ordained by the gods that a woman should only have one husband. However, she submits to many and it is therefore certain that she is a courtesan. It is my view that there is nothing surprising in her being brought into the sabha in a single garment, or even if she is naked. In accordance with dharma, Soubala has won all the riches the Pandavas possessed, including her and themselves. O Duhshasana! This Vikarna is only a child, though he speaks words of wisdom. Strip away the garments from the Pandavas and Droupadi.”
Debroy, Bibek. The Mahabharata: Volume 2 (pp. 234-235)
K M Ganguli translates it as unchaste woman:
Draupadi had been mentioned (by Suvala) and approved of as a stake by the Pandavas. For what reason then dost thou yet regard her as not won? Or, if thou thinkest that bringing her hither attired in a single piece of cloth, is an action of impropriety, listen to certain excellent reasons I will give. O son of the Kuru race, the gods have ordained only one husband for one woman. This Draupadi, however, hath many husbands. Therefore, certain it is that she is an unchaste woman.
And so does M N Dutt.
As to your other question: Was he right to say so?
Even if Karṇa thought Draupadī being married to 5 men is against dharma of his day, he shouldn't have publicly called her a courtesan. To this effect we have Manusmṛti 4.138 saying:
satyaṃ brūyāt priyaṃ brūyānna brūyāt satyamapriyam |
priyaṃ ca nānṛtaṃ brūyādeṣa dharmaḥ sanātanaḥ || 138 ||
He shall say what is true; and he shall say what is agreeable; he shall not say what is true, but disagreeale; nor shall he say what is agreeable, but untrue; this is the eternal law.—(138)