I don't! I like Chopper, I feel a sort of connection to him I don't with most of the others, he's just so pure and good, and I don't think he's useless or boring but he could definitely stand to shine a bit more as could Robin
Chopper looks so bad only because he's lost all his uniqueness within One Piece's ever expanding world:
Being a doctor: not only is he not the only doctor hanging out with the crew now, but his accomplishments have been horribly out-classed by Ivankov and Law. Plus he's the doctor for a Captain who's now immune to poison (mostly) and a first mate who regularly loses more blood than is in a blood bank. Plus when there is a medical crisis, it's always something Chopper can't solve. We're far removed from the days when the crew freaked out about a fever.
Being an animal: The Minks completely outshine Chopper in every way. Even their Sulong form makes Monster Point look weak. Speaking of which...
Chopper's Devil Fruit/Points: Monster Point has been nerfed (Luffy easily one shotted it and it's done nothing but fight nameless enemies). Chopper has somehow lost one of his points for everyone he gained (why??) and they now all look like shit. Well except Horn Point. That looks so dope it should just be his default form. Plus it shows how much Chopper has grown as an adolescent! But I suppose merchandising would've thrown a fight if Brain Point didn't remain his standard form.
Speaking of which, Brain Point's "Scope" really needs to come back. How cool would it be if during this arc Chopper used Brain Boost to analyze an enemy's weakness, used his medical knowledge to know exactly where to attack, and then used all his points to land a devastating blow.
My favorite thing about Chopper is that SBS where Oda said he intentionally made him "ugly" initially because he didn't want him to become a mascot character that exists purely to sell merch as he hates those sorts of characters.
I have literally never heard of anyone using Ivankov's abilities to downplay Chopper...
As for Law, the issue for most people is more that Law's powers are simply cooler than Chopper's. The way it flawlessly ties well into his profession, and even his epithet "Surgeon of Death", don't help either. He's like the Mihawk to Chopper's Zoro. Nothing can really be done about Law being cooler, but people can at least take solace in the fact that Law does not have the same dream as Chopper. Otherwise then it'd get truly awkward...
Though I get why Oda didn't give an ability like that to the doctor of the crew (aside from it being broken period). If you're already at a point where you can cure incurable diseases (The Amber Lead disease), it kind of undercuts the dream of wanting to be able to cure anything if you already have what is essentially a cheat code.
Luffy being largely immune isn't an issue at all. It's not like he was getting poisoned every other arc after Chopper joined, where Chopper's regularly providing him with an antidote. The most notable instance of Luffy dealing with poison happened in an arc that Chopper wasn't even a part of. There are other members of the crew anyhow if Oda felt that strongly about... poison, and in Zou he saved an entire country that was hit by poison gas, so it's not like he needs Luffy for those kind of feats.
Also if Queen is indeed a Scorpion Zoan user, then he will probably be helpful for that.
Fishman Island had the blood transfusions, Zou had the whole country and Zunish itself. On the way to WCI, he could've fixed up Luffy, but he literally didn't have enough medicine on hand. Though of course the meta reason is to introduce Reiju's powers. Same for Law too as a Supernova getting his TS debut and shine. Actually while Law got the spotlight for fixing the kids up, it's a shame that people completely overlook/forgot the minor subplot of Law asking Chopper to go and find the composition of the drug to begin with.
That's more a case of Luffy having become a loooot stronger, with him having used the strongest attack we've seen from Luffy at that point in the series(Gatling version aside). Also Franky was controlling his body, not him, which probably didn't help. You are definitely right however about it becoming a form that he uses way too often. Like damn, the dude felt the need to use it just to take down a crocodile homie. Tragic. It wouldn't be as big a deal if he at least had the cooldown removed, but when that's still there, it's just disheartening to see it having become so trivialized. Also half the fun was him being a berserker in that form, it doesn't have that same kind of menace now that he's fully controlled it.