Agree with Larry, it’s the angle of that top surface, most likely, and about using narrower lines if possible (maybe even a 0.2mm nozzle). You’re running into FDM/printer limits there. But, you could try going lower than .08mm layer height… check out Minimum Layer Height Limbo: How low can we go? I’ve gotten some really smooth results with a .04mm height (on H2C).
If you care more about the top being flat than the bottom being smooth, another option may be to tilt the whole model a bit so the top is horizontal and part of the bottom is on supports/raft. But that may/will affect the other surfaces as well, so maybe a deal-breaker.
There’s also a custom version of Studio out there somewhere with an experimental “Z-anti-aliasing” feature, which is supposed to help in such cases. I’ve never tried it myself. More infos here: Anti-Aliasing for FDM 3D Printing Is Finally Here
HTH,
-Max