I think to some degree people are overthinking this.
It's a shonen manga. Chopper is one of the main characters. He's going to need to be good enough for the end of series fight he will be slotted in for. He's going to be competent combat wise. At the moment, he's able to exchange a blow against Queen and get some blood flowing. This is more than many people can do. Yeah, he probably isn't going to penetrate upper half of the crew, but he's already in the upper percentage of pirates... At 17.
Also, come on. let's be real. Chopper has the
highest Devil Fruit mastery of all regular zoans. That should be celebrated. What started off as a drug induced modification became a permanent one. No one else has access to a total of six forms, all with dramatically different functions, plus the psuedo-awakened. (Callling it psuedo-awakened due to the need for the rumble ball).
Or Brook. He arguably hasn't fought as much as Chopper has, so maybe he's meant to be bottom tier alongside him, considering how he played second fiddle to Robin when facing Black Maria too.
Brook. The crowd control, hallucination, high-speed attack, elemental conjurer, anti-soul combat specialist.
The man with the distinction of attacking Big Mom of his own initiative more than once, and cutting her soul. Who, when he was "playing second fiddle to Robin" took down a whole bunch of smile users and a giant with one attack.
This is the man you suggest is in the same "bottom" tier as Chopper?
Getting lost in the sauce man.
The way I see it, Brook is solidly in the specialist middle tier of the crew along with Franky and Robin. And you know what? Chopper should really be in the middle tier.
Right now, the bottom of the crew's most defining characteristic is their constant desire to cry and run away. Forget strength levels. Their antics are the greatest testament to their weakness. Chopper graduated from that long ago.