Honestly, I think Kadri has done very well this season all things considered.
First of all, remember that early season injuries/contract disputes had Naz stuck playing with a rotating cast of wingers, most often being the likes of Brown & Lindholm. If anything is going to absolutely kill your chances of scoring a goal, that is it. He only had Nylander on his line for a brief time, and unfortunately, they couldn't score a whole lot, even though they were generating tons of chances. I guarantee that if he had a full season of Nylander or Marner on his wing, he would have hit 30 goals again. Even now, he is playing with Marleau, who has killed offense this season, and Kapanen, who has great speed for transition play, and a good knack for scoring, but is far from a talented set up man for Naz. In 16-17, he had Nylander for 1/2 the season, and in 17-18 he had Marner for half the season (plus Marleau was much better).
Secondly, he only has 4 goals on the powerplay this season. Compare that to 12 in each of the previous two seasons. Frankly, the new PP set up has a) not been as lethal as last season's, and b) does not utilize Naz in the high slot nearly as much. Most of our PP goals have been JT digging in front, or Matthews sniping from the left circle. That little kick back into the high slot has not been used much, and Kadri has been a bit of a ghost at times on that unit (personally, I'd love for a right handed shot to be on the 1st unit for a 1 timer option in the slot, and throw Kadri onto the 2nd unit in a more active role). But if you add an extra 8 goals to Kadri's totals, he's suddenly at 23 in 67 games, with 47 points and we probably aren't having this conversation.
Thirdly, he's had his own struggles. He was injured for a while, and has missed games. He's also had a bad shooting percentage, which is just purely luck. He's shooting at 9% this season. His career is 11.6%. The last two seasons have been 13.6% and 15.2%. His shot rates are down in total (which is probably due to the first point, lack of good wingers), but he's still on pace for a 200+ shot season over 82 games. Substitute in last year's shooting percentage, and he would have 25 goals this season and 49 points. Over an 82 game pace, that would be a 31 goal, 29 assist, 60 point season. Pretty similar to the last two seasons.