Yes. Every feature is free, with no login, no ads and no paywall. It runs in your browser using Sleeper's public league data, so there's nothing to sign up for.
Type your Sleeper username, pick a league, and you'll get power rankings, playoff odds, a luck index, lineup grades, trade grades, waiver targets and all-time league history.
No. League Lens only reads the public league data Sleeper already makes available. It never asks for a password and stores nothing about you.
No. It's an independent, fan-made tool that uses Sleeper's public API.