Usually, the ROY is the #1 overall pick, who typically goes to a poor team. So the team often fares poorly.
But some years, the #1 pick is clearly the best player (Diana, Candace, Tina, Maya, Nneka, A'ja), regardless of what the team's record is. The team that "earned" the #1 pick by having a poor record the prior year can fare much better when the rookie is really good.
I agree with UConnCat's point that it's easier and less meaningful for a player to put up big numbers on a bad team than to put up moderate numbers in helping the team to win.
A huge reason why those teams improved so much after landing the top pick is that they also got back a superstar who was injured or sat out for most of the prior year. Not having their star led to an awful record, which in turn allowed the team to land a #1 pick. Add in the healthy superstar the following year plus their #1 pick and the team's record improves immensely:
Candace Parker- Lisa Leslie sat out 2007. With both it was a 10 game improvement
Nneka Ogwumike- Candace missed half of 2011 season and wasn't healthy when she came back. With both in 2012 it was a 9 game improvement.
Maya Moore- Augustus was injured in 2010 and still recovering from an ACL. With both her and Moore it was a 14 game improvement.
Tina Charles- #1 pick was traded, so she joined a pretty decent team (Connecticut went 16-18 the year prior). With Tina it was a 1 game improvement.
Brittney Griner-not ROY, but Penny Taylor/DT sat out in 2012 and their record was terrible. Lots of speculation that Phoenix was tanking to land BG that year. With all 3 it was a 12 game improvement
DT-Phoenix didn't have a superstar in 2003, but they acquired Penny Taylor with the first pick of the dispersal draft which was obviously a massive addition. With both it led to a 9 game improvement.
Wilson is the exception of being picked by a bad team that didn't have a big time player join, but the Aces still missed out on playoffs in 2018 and finished 9th in the league. Worth noting though is A'ja did make a big impact and the team won 6 more games.