So you would hope it is high impedance and can easily be driven - but you are really seeing the impedance of the USB chip output pin.
A real WEMOS D1 mini has a 500 ohm resistor in series so you can overdrive that output, but many of the clones omit this series resistor, making the Rx pin unusable.
But do not despair!
You have two options:
SoftwareSerial is a library that implement UART on any pair of pins you choose. Pick two unused pins and connect your serial there. 9600 baud is quite within the capability of SoftwareSerial.
Or, ESP8266 has a neat feature that can swap the TX and RX pins away from GPIO1 & GPIO3 to GPIO13 & GPIO15. So you connect serial data to GPIO13 & GPIO15 and use the command Serial.swap(); to swap the pins.
But beware, GPIO15 needs to be pulled low during reset for the ESP8266 to boot. So don't do anything that pulls GPIO15 high during reset
I hope that helps