I had 61 dwellers and never even got any such message (im at 60 now cus someone died in a molerat attack or something and i missed his body somewhere)
However, from what i heard is that you can get deathclaw attacks after 60 people. So using logic, i would assume that the message that had a picture of a deathclaw was a warning that now with over 60 dwellers you can get a potential deathclaw attack, which is increased by how many times the vault door is opened and using the radio rooms.
The only deathclaw message I remember seeing said something about deathclaws attacking more often when the vault door is opening frequently. Maybe this is what you saw?
The only pop-up message i encounter is something like "would you mine to rate Fallout Shelter?"
It seems that the "Fallout Shelter Update" message keeps poping from time to time on many devices. Might be that since it has a Deathclaw image.