Fetterman was a good candidate, until his stroke. Studies have demonstrated that stokes often lead people to adopt conservative values, and that seems to have happened to Fetterman.
However, we should pump the brakes on his resignation. He is still a Democrat, and he adds to the close minority that Democrats have. He fulfills a similar role as Manchin did in the Senate. He may support the Republicans now and then, but he still gave the Dems the majority, which kept the Dems in charge of all committees, including investigative committees.
The Dems have the possibility of gaining the majority in Congress with just a few more seats. Mike Waltz' seat is up for grabs in Florida, and it was considered a safe Republican seat, but it is being challenged by Josh Weill, a Dem who is running a vicious Republican style campaign, and the Reps are extremely worried it might flip. Weill has raised 10x the money of his generally disliked Republican opponent, and still has about 10x the money on hand for the final stretch (April 1 is the election), versus about $100K for the Rep. Weill commercials, which look and sound like Republican attack ads, and never mention that Weill is a Democrat, are flooding thebairwaves, with very little response from the opponent, whose entire campaign is based on supporting HitlerPig. A few flips like that over the next year or so, and the Dems might get the majority.
Of course, Weill is still at the mercy of the Florida election system, which is fully rigged. I doubt DeSantis will let Weill win. Sorry to piss on the parade.
The bottom line is that Pennsylvania is a bright purple state, and very unpredictable. If Fetterman resigns, he might well be replaced by Republican, putting a Dem majority even further away.