I wouldn't say that she is still regarded as a CURRENT superstar. But she was one in the past, which means people have hopes and expectations that she will be again, or that she has the potential to.
Look at it this way -
Her singles:
A Moment Like This - #1
Bleeding Love - #1
Better In Time - #2 (300 copies off of #1, that still amazes me to this day!)
Forgive Me - #5
Run - #1
Happy - #2
I Got You - #14
Collide - #5
Hurt (not even a single, not sent to radio etc) - #8
Her albums:
Spirit - #1, sold 6 million+ worldwide
Spirit Deluxe - #1, sold a million alone, making Spirit total 7million+
Echo - #1, sold 1.5 million+ worldwide (Sony say 2 million but I'm not sure)
Her Tour DVD:
Leona Lewis: Live at the O2 - spent 10 weeks in the top 10 with ZERO promotion. It debuted above Beyonce's I Am...World Tour DVD, and Beyonce was huuuuge at that point here. It had sold 114,000 after 5 weeks, and spent 6 more weeks in the top 10 after that, so have a guess how much that sold!
Other points:
-She completed a sell out 20 date UK arena tour, off the back of an underperforming album.
-She sang the theme song to the biggest movie of all time
-She performed at the Beijing Olympics closing ceremony to over a billion people.
-Her song 'My Hands' was chosen for Final Fantasy game in an underperforming era
-Her songs were chosen for other high profile movie soundtracks (Sex And the City 2, Precious)
-Was chosen to do a US Cotton commercial/advert the underperforming Echo era -
http://www.youtube.com/watch?v=ncFkelvp2ho
-Artists such as Beyonce, Whitney Houston (who called her a 'mini-me'), Adele, Diddy, Ne-Yo, Akon, Jessie J, Justin Timberlake, Nicki Minaj, Katy Perry and many more have all commented on how great they think she is.
All of that shows that she once was a superstar, and still has the potential to be, a big star. Poor management, poor single choices and delays have killed her buzz, so no, she's not one now. But personally I think she still could be!