I do not recall ever being able to pinch to zoom in Apple Mail to increase the size of the message body.
CMD + / - works to temporarily zoom in and out. I agree, would be a nice to have feature.
There's a variety of Accessibility Zoom options but that tends to not be what people want.
I tested a third party App called BetterTouchTool and it works, I was able to configure a pinch and zoom option but had to repeat the gesture for every incremental step because all it was doing was sending the keystrokes for CMD + / - to zoom each time I pinched or reverse pinched. It's easier to just hit CMD + / - but perhaps you could find some alternative configuration using this tool. It's quite powerful. But you aren't going to get a smooth zoom in/out unless Apple adds that specific feature to Mail.