You'll have to read the opinion, which is fairly long, to get the gist of it. It really isn't as simple as asking "should Biden's win in Wisconsin be overturned?"
Trump argued some points of Wisconsin election law that he said necessitated throwing out absentee ballots. Basically the arguments centered around election clerks completing addresses or other information on ballots that were submitted not in conformance with what the statute says, a claim that ballots collected during various "Democracy in the Parks" rallies shouldn't count, and a claim relating to some declarations last spring by election officials that the Covid stay at home orders constitute "indefinite confinement" which then triggered certain requirements.
An examination of those points reveals a couple of things. First, yeah Wisconsin's election process as handled this year was completely screwed up and haphazard, probably no big surprise. They basically winged it, which of course leads to all sorts of challenges. But the bigger problem, and one that Trump could probably never overcome, is that the remedy he is looking for (just tossing the absentee ballots) was never going to work. It's impossible for him to figure out and prove which ballots were cast contrary to the rules, but his remedy that therefore
all absentee ballots be tossed is clearly wrong.
It looks like the primary dispute at the Wisconsin Supreme Court was the way in which the Court chose to decide the case. The majority went with the doctrine of
laches, which is sort of an archaic and rarely used notion that if you unnecessarily delay in pursuing your remedy, your claim may be barred. It was the majority's way of punting the decision out of the Court without having to go through the troublesome analysis of what was truly a mess in the Wisconsin election system.
The dissent felt that the analysis should be undertaken, if for no other reason than spanking the election officials and election commission, but I think one of the dissenters even noted that doing so doesn't mean that Trump had any chance of winning on the merits. They just wanted the merits discussed.