It's impossible to tell how much of the posts are bots but if you look at the default subreddits I'd say that it's definitely noticable.
Reddit is a popular grey marketing area - be it shilling products or political propaganda. Lemmy/Federation hasn't reached enough mass for this to be a real problem yet but it'll happen eventually and Lemmy is an easy target right now. I used to work in bot detection area and modern, well made bots (the ones you should worry about) are essentially indisguishable from real users but script kiddies can be an issue too.
The only real way to fight bots is to reduce the incentive which is more of a cultural thing - people have to call out shills and a more transparent platform definitely helps.
Sorry for the rant but I think bots/spam will play a big role if federation ever reaches the point where there's enough eye-balls for shilling to be valuable.