This would be true but for one thing; the New Mutants were not a superhero team, still less a pro-active superhero team. They were a bunch of kids who were being trained in the use, control and extebt of their powers. More often than they found themselves in outre situations, and using their powers to help others, but that was usually because of their affiliation with Xaviers, not because they were the Teen Titans.
And if we're talking vulnerability, note that Mirage, Karma and Sunspot only had normal levels of bullet-proofness. Karma in many ways was worse, as using HER power often left her original body comatose. So she has the power of combat narcolepsy!
If being an X-Man depends upon having martially useful powers, then the whole point of the X-Men is lost. Doug was just as much a mutant as Wolverine or Gambit or Cyclops, a Sentinel would target him just as much as it would those three examples.
Putting on the uniform didn't make you a field team member, it identified you as a student, and Doug was definitely that.
Plus of course, his pal Warlock was there for him up as body armour until 'Lock's completely out of character, improbable and downright weird decision to leave the New Mutants to run away from home with the ever irritatingly petulant Sunspot, without even a word to Doug. Almost as if it had been planned to hammer home how mortal Doug was, when we already KNEW that.