I wouldn't jump to conclusions about "should be"--remember that they're climbing from what seemed to be single-digit/fourth-place purgatory destiny (though Nanos actually jolted them *down* a week ago from what had been mid-teens for them; maybe a sort of early-race herding of the pollsters, as other hitherto NDP-unfriendly pollsters have seen them rising back into double digits). I don't think even the NDP is under any illusion that they entered this election in rough shape, or that their real goal was to come out of it all with at least some dignity intact. And so far...Jagmeet's been overachieving on the "dignity" front. It's still to be seen whether this leads to concrete results over the course of the campaign; but given where they've been coming from over the past year or so, it's almost like...so what if it's less than 20 seats. 20 seats might be less than half of what they got in 2015; but Jack Layton got 19 seats in 2004, and Alexa McDonough got 20 seats on 11% of the vote in 1997. By comparison, when this campaign began, it looked like the NDP might be headed for a 1993 Audrey McLaughlin-style disaster of 7% and single-digit (or even zero?) seat numbers--*yet that still seems to be the conventional wisdom among some in this thread*.
In some ways, I might even compare Jagmeet's campaign performance thus far to Mike Harris's provincial performance in 1990, when the provincial PCs were basically broke following their 1997 disaster and the federal PCs at the nadir of their popularity not helping matters--yet with a lean "taxfighter" platform and not-bad performance on the hustings, he led the PCs to more seats on fewer votes and a foundation for the Common Sense Revolution five years later. So, still third place, but it was a pretty good salvage job that allowed him to live to another day as leader.
Maybe the problem with your argument is that you're using a "do very well in Barrhaven" argument--which did come to be in last year's provincial NDP "Ottawa surprise"; however, keep in mind that over the four-five decades or so since Barrhaven started to be developed, the best the federal NDP has done in a riding encompassing Barrhaven has been 16%, in 2011. So if you're using that particular barometer or benchmark, well, the NDP never really had any business existing, I guess...