Nozzle engines absolutely have to account for the external pressure. The optimal pressure as the exhaust leaves the bell should be as close as possible to ambient for full thrust.

If the pressure at exhaust is higher than ambient, the exhaust pushes outward against the ambient pressure and you get huge exhaust plumes, and lost efficiency.

Conversely, if the pressure at exhaust is lower, the ambient pressure pushes the exhaust inward into shock diamonds[1] and you, again, lose efficiency.

Engine bells specifically yield their max efficiency at one external pressure/altitude. The reason you see shock diamonds is most often from ground-level testing (or takeoff) of engines that perform best at altitude.

[1] https://en.wikipedia.org/wiki/Shock_diamond