Signature is not responsible for spam.
We have very high ranked members who wear signatures of various projects. Spamming is common with low quality users, regardless of forum rank, signature etc.
First of all, I strongly disagree that the spamming problem bitcointalk has isn't related to signature campaigns. I highly doubt we'd have people copy/pasting other people's shitposts if it weren't for the fact that there's money to be made by doing so. I've never seen that being done on any other forum I've been a part of. And I also doubt that there would be so many spammy, gargabe, nonsense replies being made in English by members who clearly have no command of the language if it weren't for signature campaigns.
But the problem isn't with the campaigns themselves; it's with the low-quality posters who get accepted into them. There are indeed some senior members who are complete shitposters (
Amph comes to mind), but the low-ranked ones outnumber them by a huge margin from what I can tell. I don't know if there's any way to quantify my claim, but last I checked Bitcoin/Altcoin Discussion had been overrun by mostly newbie spammers. My point is that the problem doesn't come mainly from the Hero/Legendary accounts.
As far as OP's point, I'm not sure I agree with it or not but it's an interesting thing to think about. People definitely become more excited and want to earn as much bitcoin as they can when the price is rising. I think that may translate into more posts and more spamming being done during bull markets. It's too bad it doesn't resolve when the price of bitcoin has been battered down to the grass, like it has been recently. There have been as many shitposts as ever, and I'm sure the sections of the forum I have blocked haven't improved much. My guess is that they're probably as bad as ever.