Today I'm using mastodon, but I primarily access the 'verse via #Friendica. Friendica supports a "title" field that shows up as a content warning when accessing such a post from a Mastodon instance. Would they consider most such posts abuses of content warnings?

Yes, this is abuse IMHO. When "title" is mapped to "warning", this is clearly a semantic break. A title is per se completely different from a CW if you take CW seriously. However, it might be the case that we end up with "titles" all over Mastodon and rename this feature somewhere in the future because many people do seem to switch. Bad for the people who rely on real CWs IMHO. YMMV.