It's not JUST that the Vita is marketed as a console experience, it's also that they're taking a genre that is typically experienced on a console. When we play a game in a genre we inevitably compare it to other games we've played in that genre. With FPS games, there are so few on handhelds that were worth playing, so our frame of reference is entierely handheld games.
That's another part of the problem with trying to do console gaming on the go. A lot of people actually DO play their handhelds at home alot of the time. Many gamers might find themselves in the circumstance of choosing between Killzone Mercenary and Shadow Fall. If Mercenary is not either as good, or somehow distinctive, that could be a problem.
Sony's marketing exacerbates this. Should we take PR seriously? Well, yes, I think we should. If that's what Sony is promising to customers, that's what they should expect. Of course PR people make BS statements all the time (PS2 will have toy story graphics, XBox 360 is like playing a lucid dream, all multiplatform titles are significantly better on Wii U), but just because they do it often does that make it ok? If they lie often enough should we just stop calling them on their BS? If they promise something and don't deliver, that's not ok.
Of course, what's getting lost in all this is that 77/100 is a pretty good score and it could go up. When Wonderful 101 was getting those kind of reviews, it didn't make me want it any less. Mercenary may not be the savior of the Vita that many were hoping for, but it sounds like a pretty solid shooter that will be especially good for people who are constantly on the go.