I mean, I guess if I'm feeling generous to the Ian Happ haters, not sure why, he's had 1100 more PAs than any other player in a Cub uniform from 2018 through today, during which time they've won exactly zero playoff games. Over his career, which is basically that stretch plus 2017, he's hit in the 3 spot in the lineup more than anywhere else.
It's not exactly logical (some would say dumb) to be mad at Ian Happ for the organization not putting better hitters in the lineup for what is coming up on an 8 year stretch. Ian Happ, as a 20 fWAR guy over essentially 7.5-8 seasons is plenty valuable, he's made like $85m in his career so he's not overpaid, etc. But he probably shouldn't be the best player in your offense over that type of stretch, and he has been (Hoerner actually just passed him in total fWAR from 2018 on, in significantly fewer games, because Nico Hoerner rocks, but that's obviously heavily defensively driven).
Basically, people are probably right to be frustrated with the total team output since Happ has been here, and Happ is overwhelmingly the guy they're likely to have seen while watching said frustrating team.