I've only done one middle of may trip - started out May 12th in 2000.   Fishing was ok, as I recall, but likely to be slow - probably depends on how early ice-out occurs.  Weather in general tended to windy and colder, as you'd expect.

portage trails will most likely be wet and muddy and could be under water in places if the water is high;  and you'd better carry a saw for clearing a path thru, over, under or around the blowdowns - if it was a heavy winter, there will be lots of that - portage clearing crews are probably just starting to get to work.

temps will run to below freezing, most likely  -  not gonna be very hot and wet-footing is cold work, especially on the trails if there is a lot of melt water around.

of course, with global warming kicking in, it could be a lot nicer than that

likely you'll be in the park before it "opens' for the season, so you'll have to plan on self-serve payment at the ranger station (might be able to pay ahead with credit card ? - not sure) so bring cash to put in the drop box.