hi! while you’re technically correct, i’m sure people aren’t able to tell the difference between 17ms and 16.666…ms (60fps) or 33ms and 33.333…ms (30fps).
after all, when movies in 24fps are converted to 25fps pal tv broadcast, they are simply sped up - that’s ~4% increase in speed. still, it is generally considered better option than to repeat single frame each second, because it is more important to have a smooth visual experience.
another thing is: can aseprite preview 60 fps correctly? i don’t know, i don’t have a way to measure it. what i do know, however, is that after effects are often struggling with 50 or even 25 fps when the project is loaded with data and effects - and that’s on workstation with high end gpu and 64gb of ram. same is true for video players.
so, while it would be possible to make such script, i don’t believe you need it. just round the frame duration to target framerate and keep proper amount of frames per second and you’ll be fine. any difference between aseprite preview and exported video footage won’t be noticeable.
there’s also a question why you would even need to animate in 60 fps, but that’s another topic.