Stealth is mostly meant as radar stealth. It offers concealment of the engine's round nozzles from radar, but in serrated ones the shaping gives some degree of low-observability against IR too (nothing significant tho, IRST can track it fine).
Mass entrainment around the boundary layer is not directly related to that, it'll actually happens more if the aerodynamics is bad!
Here how; in case of inferior streamlining, low pressure is created around the tail end, drawing air from the sides. That cooler air from sides mixup & form a screening layer.
But such low pressure is not good for aerodynamics & generates unnecessary voetex at the wrong place, creating drag.
But that's again another tradeoff.
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