If the user is MITM'd, what's preventing the attacker also replace the signature to verify against?
EDIT: I see what you mean. radicaldreamer stated that a malicious root certificate is installed, but signature validation wont help there. But, it will help when downloading from mirrors or HTTP.
The currently running (trusted) executable downloads and verifies the signature of the binary. Then after verification you execute it. If your trusted binary is validating invalid data then you've already messed up somewhere.
The starting point was: an attacker has control over the system so that “the end user could be MITM'd already with a root certificate maliciously installed on their device”.
In that case, there's nothing “trusted” on your machine anymore and all bets are off. Doing signature verification in app instead of relying on HTTPS is security theater[1].
[1] or it could be “defense in depth” but that's an argument I'd only accept from someone who really understands what they're talking about, and only in a context where everything else being being done properly. Most of the time “defense in depth” is just an argument for the security theater.