Used vehicle with low MPG average on DIM.

When looking at a used vehicle ad and scrolling through the photos, say you see a photo of the vehicle’s DIM with an absurdly low average MPG. Is this a red flag on mechanical issues and/ or CEL’s? It tells me they disconnected the battery in an effort to reset something.

I saw an ad for what appeared to be a used nice low mileage Tacoma. The DIM displayed ~5 mpg on average. Is the dealer hiding something?