I've personally yet to encounter anybody who holds (or read any arguments in favor of the view) that Russia is outright winning this conflict in any tactical or operational sense, beyond perhaps a very marginal measure - without it becoming immediately clear that the person making them had major gaps in their knowledge, and/or strong biases (as revealed by reciting standard narratives as to the supposed causes of the war). Or without it emerging (as with the sibling commenter) that they just don't seem to give a fuck either way.
But this is of course a different topic.
Mearsheimer is a prime example of someone way too ideologically biased and beholden to broken narratives to be taken seriously.
For just a bit of - at the glance similar - but much, much more thought out and balanced stuff, I would recommend anything at all by Steven Kotkin. He speaks with folks from Brookings quite a bit. His experience is unparalleled and world view is considerably more nuanced
"shouted down as a traitor" does not get at the crux of the problem I see. The problem is that anyone who thinks as follows cannot form an accurate opinion about the likely outcome of the war:
Anyone who argues that Russia is winning the war is probably a Russia sympathizer, and a Russia sympathizer shouldn't be trusted and consequently we don't have to consider his argument.