Mearsheimer is extremely pro-Kremlin pundit though, he's even welcomed [1] at Valdai Discussion Club ("The Valdai conference is closely linked with Russian President Vladimir Putin, who has met with the participants of the Valdai Club’s annual meetings every year since its founding." [2])
He is neither pro-Kremlin nor anti-Kremlin. Mearsheimer is a political scientist with 50+ years of experience. He relies on pure facts and explains why things are the way they are. Nothing more, nothing less.
He tells you the facts that you want to hear, that is all.
His view is Kremlin-apologist.
You are not in Ukraine, you rely on second-hand account. So for any 'facts' you have to rely on someone. Ukraine is a complex topic, and 'things the way they are' is extremely hard to get, there is an ideological lens to everything.
His lecture was shared recently on some Ukrainian forums and widely derided as just repeating Russian state propaganda talking points.
[1] https://twitter.com/m_suchkov/status/1483765876310511617
[2] https://en.wikipedia.org/wiki/Valdai_Discussion_Club