Don't buy into that. Cap's numbers are no more important now than they were prior to the release of Green Lantern. This film won't be a critical success, but it still has a chance to be a commercial success. Critics tore into The Hangover II and that walked away with a healthy $488 million gross (off of an $80 million budget). Green Lantern won't walk away with that kind of budget-to-gross ratio, but if Superman Returns could hurdle the gap between it's budget and profitability, then I see no real reason why Green Lantern cannot do the same.
This fan is most disappointing to comic fans, but for the average movie goer, this film is filled with snappy one liners, mostly good looking special effects (Hal's suit being the glaring exception) and a few solid scenes of action and adventure. $52 million may not have matched Thor ($67 million) or exceeded First Class (which made $55 million on its opening weekend but has long since pulled into profitability) but those are strong numbers. This isn't like when The Spirit debuted and mustered $6 million and 9th place on opening weekend. Green Lantern is number one right now. It will get knocked down to number two by Cars next weekend, and number three the following week by Tansformers, but it will remain fairly healthy. There is always the overseas market.
As for the idea that the comic book movie is now going bust, I really doubt it. This genre has endured legitimate box office failures such as The Phantom, Jonah Hex, The Spirit etc.. None of those has harmed this genre. All of these detractors will change their tune next year once the new Batman film comes around and being into comic movies is trendy once more. Comic book movies are the new action movie. Gone are the days of Arnold and Dolph. That leaves a cultural void to be filled. Super hero films seem to be doing a bang up job of filling that void.