Accepting the endorsement of Liz Cheney seems cynical, counting on most voters to have short enough memories to see Liz Cheney, only in terms of the last thing she was getting headlines for, ie., being a Republican that took a hardline NeverTrump stance. It seems like Harris is also trying to come off as being gracious. It goes along with the whole moderate outreach the Harris-Walz campaign has been doing. I'd also say praising Dick Cheney was laying it on way too thick. She could have just accepted the endorsement and left it at that... or just commented generally about "uniting across party lines to save Democracy" or similar.
The idea I think, is to embrace these "permission" endorsements from prominent Republicans, with the perception being that the endorsements give Republican voters "permission" to switch sides and vote for Harris to express their disdain for Trump. I'm skeptical that this kind of Faustian bargain is even going to have any impact. I doubt many voters who admire Dick Cheney would be inclined to vote for Harris under any circumstances. Obviously its going to be off-putting to some of Harris' potential voters, but I don't think it will be enough to induce a person who was committed to voting for Harris-Walz to suddenly say
"Oh Harris accepted the Cheney's endorsement? That's it I'm not voting for Harris anymore!" I'm guessing that most Democratically inclined voters for whom this is the most upsetting were already not voting for Harris anyway.
Either way, I agree that it kinda sucks, in that I think it was unnecessary. It's not going to add any votes, and it just makes committed Harris-Walz voters have to cringe/facepalm.