Under what circumstances are you able to rate a Mii as fantastic or not? There are two 3DS in my household that constantly streetpass but rarely ever do the Miis get to rate each other as fantastic or not.
In my experience, you get rating opportunities if one of the following three things occur:
- The second time you meet someone.
- When you meet someone you've met before and they've changed their hat.
- An unknown criteria when you meet someone past the max number of times (currently 999). This one seems to have a pattern (maybe every 10?) but I don't know what it is.
My understanding is that you can only rate a Mii as 'fantastic' after you've previously seen it at least once - the first meeting doesn't allow a rating to be given.
If the 3DSes are passing through the same place repeatedly (as mine does since I take it on the train to work) then you're perhaps more likely to meet the same Miis multiple times and therefore rate them.
You are able to rate a person the second time you meet a person's Mii. If you rate them "Fantastic!", then they should receive that rating on the third time you meet them.
If their Mii changes their appearance (e.g. wear a new hat), then it allows you to rate them again. However their Mii can only receive 1 "Fantastic!" rating from you.
Now I have heard that if they change their StreetPass Mii for a new one (created in Mii Maker), then you can rate that new Mii on the second encounter and they will receive it on the third encounter (i.e. getting a second "Fantastic!" rating from you). Note: Having not seen that myself, I don't know whether changing your own Mii would also let you give a second rating to someone's Mii. I'm guessing it might work though.