Honestly I think differentiating the wiki and forum favicons by having the forum one stay 3D is a good idea. Renders do keep improving, but at such a small scale I think the differences are too negligible to be visible. There's not gonna be another jump on the level of the one from the Wii to Wii U rendering styles, unless the series actually makes a drastic change to its art direction which I feel is unlikely.
The larger icon used for the unread indicator, that I can more get behind making 2D. Would be neat if we just straight up used an SVG for that. But tbh the favicon being 3D I found useful for differentiating wiki and forum tabs, since the wiki's is 2D, and again the updated render concern becomes negligible at such a small scale imo.
(it may be just me but I also wouldn't be opposed to making it Mario's face again, i lowkey miss that)