Hmm... only really whenever the Northern Wei is directly involved in events in the south, such as during the Wei-Song wars. Otherwise, I think they could have been a lot worse. They left out a lot of questionable anecdotes in the Songshu about Northern Wei, for example.

When it comes to the Wei-Song wars, the coverage is generally favourable towards Liu-Song. Which is not really surprising given that the ZZTJ appears to take the view that the Southern dynasties are the "legitimate" dynasties during this period. I think this could be related to the position of Sima Guang's Song at the time, which were being threatened by the Jurchen Jin to the north. So the Zhao-Song were in a similar position to the Liu-Song, so it also served the Zhao-Song to depict the Liu-Song similarly. And even then, he sometimes ends up favourable to Northern Wei at times, even if I don't think it's intentional - for example, neglecting to mention that Northern Wei only gained part of Yan and Yu provinces in 423 and saying Northern Wei gained all of those two provinces.

But I think that aside from that, the Northern Wei coverage is generally alright. The main problems aren't really bias but that Sima Guang sometimes writes things in the ZZTJ that aren't in the Weishu or Jinshu when referring to the Sixteen Kingdoms. I haven't been able to find an earlier source for some statements, but I do give him the benefit of the doubt here because there could be sources that are lost today that he could be able to draw on. Also, some of his interpretations of text in the Weishu can be a bit questionable, but I don't think it's biased.

下邳闕宣聚衆數千人，自稱天子；陶謙擊殺之。

〈《考異》曰：范《書‧謙傳》作「閻宣」。今從《魏志‧武紀》及《謙傳》。《魏武紀》又曰：「謙與宣共舉兵，取泰山華、費，掠任城。」《謙傳》亦云：「謙始與合從，從遂殺之，幷其衆。」按謙據有徐州，託義勤王，何藉宣數千之衆而與之合從！蓋謙別將與宣共襲曹嵩，故曹操以此爲謙罪而伐之耳。〉

193.H. Que Xuan of Xiapi collected a force of several thousand men and styled himself Son of Heaven. Tao Qian attacked and killed him. (de Crispigny’s To Establish Peace)

(Sima Guang's commentary in the Textual Analysis states, "The Biography of Tao Qian in Fan Ye's Book of Later Han records the name of this rebel as 閻 Yan Xuan. But I follow the account of the Annals of Emperor Wu (Cao Cao) and the Biography of Tao Qian, both in the Records of the Three Kingdoms, in calling him 闕 Que Xuan.

"But I note that the Annals of Emperor Wu in that text states, 'Tao Qian and Que Xuan raised troops together; they captured Hua and Fei in Taishan, and ravaged Rencheng.' And the Biography of Tao Qian in that text also states, 'At first, Tao Qian worked together with Que Xuan, but afterwards, he killed him and took over his forces.' Now during the time that Tao Qian occupied Xuzhou, he relied upon just behavior and exerted himself on behalf of the Han royal family. How then could he have utilized Que Xuan's thousands of men and gone along with him? It was just that Tao Qian's subordinate general(s) and Que Xuan attacked Cao Song together, and that was why Cao Cao pinned the blame for his father's death on Tao Qian and marched against him.")