Monsutatesu isn't special (scrap)

mi monsuta could be I am afraid or I am scary

musi does this too, "entertaining" or "entertained"

there are more clear grammatical ways to express either of these (the former is better as a modifier and the latter is better as a predicate)

but monsuta is not really special

context consistently tells these apart

and if you only have text, you can be totally clear in distinguishing them with pilin

mi pilin monsuta means I am feeling scared much more than it means I am feeling scary, a a a

it certainly could mean that, but mi pilin musi could also mean I'm feeling comedic

both are rather silly as interpretations even if possible

yeah, there's somewhat more ambiguity in monsuta because two related-but-separate meanings are present in interpreting it, which apply both to "SUBJ li monsuta" and "SUBJ li monsuta e OBJ"

but i don't see how context doesn't distinguish the two

and i don't see how this isn't how toki pona already works with other words, monsuta is just the strongest example of it

we already collectively distinguish musi (entertaining) from pilin musi (entertained)

relatedly, musi interjection can be interpreted either/both ways and still be correct bc of the subject you interpret to be taking musi ("musi!" -> "This is funny!" / "I am amused!"). context doesn't even need to save the day here! monsuta interjection, if it were used (i haven't heard it) would follow the same pattern ("monsuta!" -> "this is scary!" / "I am afraid!")

and you can still do these without pilin if you really are pilin't, it's just harder