Why is plus 4 points higher than the base game?
From what previews and talk I've seen of the plus version, its not any better. That the new stuff isn't that good.
Main reasons why it seems to be scoring better:
-More content. 2 new characters, one of which is super overpowered for boss fights which makes the game much easier, a new half stage, the new Encore mode that lets you switch between characters mid-game, new pinball special stage
-Aesthetic upgrades, such as redesigned menus and optional new color palettes for stages
-Option to disable the 10 minute time limit, a very handy feature for those who like to explore the zones instead of racing through them. This is particularly handy on the final zone as you can hit the time limit on that zone even if you are trying to go fast because the zone is very big and has some tricky sections
-Blue Spheres is no longer the forced special stage (some people hate Blue Spheres mode), pinball replaces Blue Spheres in Encore mode and in normal mode if you completed all Blue Spheres stages previously
-The framerate issues on the 3D special stages have been fixed on Switch (the version that has the 90 meta)
Also there's the fact that that there are less reviews for Mania Plus than there were for Mania, because it's a rerelease it's mainly critics who actually like Sonic that are playing Mania Plus, whereas standard Mania was played by a bunch of critics who weren't even Sonic fans.