FTFY
Honestly Applause did what it needed to do; it sold well and got Gaga back on people's minds and at the forefront. There were better opportunities for a lead single but it did adequately enough.
The problem was that she waited too long to debut a second single which, when scrapped and replaced by another single entirely, slowed her momentum down to a halt literally right before the album release hype was meant to take place. She started back at zero a week and a half before ARTPOP came out, and because of it only her diehard stans ended up buying it.
To most pop listeners in the world, Gaga simply doesn't have a song out right now. She should have released Venus mid-October, with the video at the end of the October, and that would lift her straight to the US promo string she's doing now. Now she's still in the middle of introducting DWUW to the world and that's been technically out for a while. If the video gets her some buzz then she'll be back full swing, but until then she's a sitting duck sales-wise.
EDIT: And yes, the song that could have benefited her the most as the lead single is Venus. It's kooky, it's infectious, it's crazy. Given the comparisons to "Roar" that would have taken place Venus would have benefited from being alternative and crazy in a way that Applause wanted to but couldn't quite hit. Basically, Venus hits hard, and Applause doesn't. Venus would have been the best possible reminder of Gaga's pinache to the public and with an epic video that song could have actually been a BR-level smash.
