The IMDb rating is reflected as Critic Rating (depending on which rating source you’re using in your library). However you cannot filter by that score – I suppose that’s mostly because each critic rating is using a different range and Plex will fallback to another system if there’s no rating in the preferred one (e.g. IMDb → 1-10, TMDb → 0-100%, Rotten Tomatoes → 0-100%).
From what I can see on my system, critic/audience ratings are only available for sorting.
Edit:
there’s a feature suggestion discussing this request → Allow Filtering By Critic Rating and Audience Rating in Movies library
