The DM attacks should be critting as expected on a scrapper. I've got a DM/Inv brute and a DM/Bio scrapper right now, love them both. Oddly, my scrapper is a pure concept build and her attack chain is nothing but Smite, SP, AirSup, SP, repeat. AirSup, of course and unfortunately, never crits but the other two do and more so since she has the scrapper AT IOs. The no crit on pool powers is an oversight that should have been corrected LONG before the snap occurred.
This may not last, however, as there's really no reason a skinless woman kept alive by blood magic couldn't use said magic for another self-heal or a major hold. But that's my one major gripe with DM these days... the best attack chain for the brute is MG, Gloom, SL, Gloom, repeat. Nothing but left-hand jutting foward over and over. Gets boring. I don't know if the dark snipe for scrappers is left or right handed.