They also may be more likely to use Do Not Track settings or similar - not sure how that affects itch metrics. Fediverse peeps tend to be pretty privacy minded.
Also, I doubt either of us gets thousands of impressions on their toots on Mastodon. So I'm not really comfortable in making definite statements about the CTR on Mastodon. I don't have any data to base such statements on. (And for the record, I think the lack of analytics on Mastodon is a good thing because it is a social network, not an advertising platform. I treat it as a platform where I can talk about my games with interested peeps but I try not to think of that as "advertising".)