I'm not Ningauble, but I can give you a fairly useful answer on this one. No, the number of mistakes is vastly reduced... quite minor, in fact, amounting to the nearly inevitable sort of things that creep in during the production of any book.
I concur with j. d.: the mistakes are for the greater part quite minor. I've compared The Call of Cthulhu and Other Weird Stories against the Arkham House editions, and here is a sample of differences I came up with (please note my use of the word differences here; I don't know whether these are real typos or not because S. T. hasn't looked on them yet. However, I suspect that the AH text -- on the right -- is correct in most cases.):
THE COLOUR OUT OF SPACE
*172.25: first begin to get thick.] first begin to get very thick.
*178.8: the anatomy and habit of squirrels] the anatomy and habits of squirrels
*178.13: leaps of the rabbit] leaps of that rabbit
*178.32: store in Clark’s Corners.] store at Clark’s Corners.
*178.36: they had held] they held
*181.8: a timid woodmill salesman] a timid windmill salesman
*182.7: locked up in the attic.] locked in the attic.
*182.14: in the stalls] in their stalls
*184.7: It must only be] It must be only
*184.24-25: to cling around the Gardners] to cling round the Gardners
*185.28: Thad had gone,] Thad was gone,
**188.25: He whispered,] [would make more sense with a lower-case "h" here
*188.31: out of everthing] out of everything
*189.28: Thaddeus already being known,] Thaddeus being already known,
*191.33: in the aërolite] in that aërolite
*195.8: poor Hero] poor tethered Hero
*195.30-31: with the gnarled, fiendish] with their gnarled, fiendish
*196.19-20: crackling, not an] crackling, and not an
I should point out that 188.25 is based on my own gut feeling: NO appearance of the story has a lower-case "h" here, not even the manuscript from what I've heard (on the other hand, the manuscript was most likely typed by someone other than Lovecraft). However, the text makes more sense with a lower-case "he" here.
Au contraire, my dear j. d. The recently released trade paperback from Del Rey is -- judging from the interior layout -- photographed from the Arkham House edition, which would make it the only Del Rey edition of Lovecraft that I can recommend with a clean conscience.His revision tales -- also worth reading, in some cases just below his best original work itself -- is unfortunately only available currently in the Arkham House hardbound volume, if you want a good edition.)