No. It depends on their home instance.
A few may go out of their way to make it easy - there is nothing stopping a Lemmy instance from requiring government ID to sign up, after all. A few may go out of their way to make it hard - there is nothing forcing a Lemmy instance to collect any data about a user. Most big instances will probably be at the same level of difficulty as tracing someone from their email address - their servers are probably logging IPs and locations, which will be a starting point for tracing identities, but not guaranteed to be "easy" by any means.
Lemmy does not have anonymous voting - https://lemmy.eus/post/182574
Each instance decides what to show on its homepage and its own moderation rules, so you are free to build (or find, if one already exists) an instance that attempts to prevent the kind of manipulation you are worried about.