Could aliens have seen dinosaurs from space?

Yes — in principle, but only in a very limited sense.

If an alien civilization were far enough away, the light reaching them today could have left Earth millions of years ago, so they could be seeing Earth as it was during the dinosaur era. The exact distance depends on how many millions of years of lookback time you mean, and the corresponding light-travel distance would be of the same order.

The big catch is resolution: seeing “a dinosaur” as an actual animal, rather than just a tiny blur of Earth, would be extraordinarily difficult. From such distances, even a very advanced telescope would be far more likely to detect broad planetary features than individual animals.

So the accurate answer is:

• Yes, in time-delay terms: they could be seeing Earth from the dinosaur era.
• No, realistically: they almost certainly could not make out an individual dinosaur.

What they’d be more likely to detect are things like Earth’s atmosphere and other large-scale biosignatures, not a clear image of a dinosaur.