I do think it's fair to say that Superman/Wonder Woman SHOULD sell better than Wonder Woman's solo book for a few reasons:
1.) SM/WW is newer.
2.) SM/WW features 2 of DC's most iconic heroes where as Wonder Woman's solo book features one of them.
At the same time, there are other solo DC books that are just as old as Wonder Woman that are selling better than her currently:
Action Comics
Aquaman
Batgirl
Flash
Green Lantern
Nightwing
Superman
And that's not counting books that star Batman solo each month. It's true that sales don't always equal quality but that's because quality is based on opinion and nothing more. I think Justice League is a top quality book while I've seen other says it's garbage. At the end of the day, the only way you can really measure the success of any book is by it's sales. Given that we are not made privy to digital sales, all we have to go on at this point are the sales of physical copies.