1) Using Skinparam
There does not seem to be a "NoteRoundCorner", but you can set the global and then override the participant like this:
skinparam RoundCorner 0
skinparam ParticipantRoundCorner 30
2) Using STYLE (the replacement for Skinparam):
<STYLE>
note {
roundcorner 0
}
participant {
roundcorner 30
}
</STYLE>