I believe that the limitation is part of the Mastodon app, and not related to the Fediverse. There might be a character limit but I don’t think it’s as limited as Mastodon’s.
Btw, Mastodon isn’t really fit for this type of conversation. Mastodon aims to replace tweeter - microblogging interaction, where Lemmy aims to replace Reddit - thread interactions.
Each comment on Lemmy will be treated as microblog on mastodon which isn’t really practical.