I may be in the minority, but I'm happy with Witherspoon and Smith-Njigba. When you look at the big boards that simply ranked players by talent, not mock drafts that tried to predict where players would go, both Witherspoon and Smith-Njigba were ranked very high.
PFF, for example, ranked Witherspoon number 4 overall this year, and Smith-Njigba was ranked number 9. As others said, these choices basically mean that the Seahawks picked the best players available, rather than picking based on need, which always results in reaching.
I personally wanted Jalen Carter, but the Seahawks weren't the only team that decided to pass on him. Most, if not all, of those teams met with Carter and were still left with questions.