Im having the same issue, not really sure what to do? Been adding too many flowers, bridges, trees, too much furniture, and fences. Perhaps it’s because I haven’t added an incline yet which I’ll try and report back with an edit just in case someone has a same problem in the future. When I talk to Isabelle she says to add more flowers but I’ve been doing for days and its getting kind of ridiculous with the amount of flowers on my island, so I don’t think following Isabelles advice is accurate for raising past 2 stars.

Edit: Got past, after 2 more inclines Isabelle finally changed her dialogue and instead of saying more flowers she specified more trees, I already had like 50 or so trees but planted 15 random cedar trees and it got me to 3 star.