Ultimately, any name is just a collection of sounds - some collections are just more acceptable in certain social circles and cultures than others.
Making up names is nothing new either, people have been doing it for centuries - I'm sure even Susan was considered pretty out there at one time.
Who knows, Tiaamii could even be pretty mainstream in a few years time - rest assured some people out there will like it, after all look at how "Brooklyn" caught on .
I can't say I've heard Tiaamii before because I haven't (not surprising I suppose if they've made it up) but as far as I'm concerned a lot of people of Afro-Caribbean origin routinely call their daughters names which do not sound too dissimilar to this one.
Good on them for choosing something different I say - after all does the world really need another Olivia, Grace, Emily, Sophie or Charlotte?