It could be, but you could also tell that to an actual tiger or someone at a costume party.
Whether this actually is flirting or not, can it be used to flirt? Or will you just sound like a confused child at a costume party?
Imagine a line that you could guarantee the success of. I think that would qualify as a spell.
Jesteś tygrysem is gramatically correct sentence, that can somhow be imagined to have some kind of usage.
"Jesteś tygrys"- could only be used in flirting, (nickname?)
Tygrysek. (Tygrysiczka, from tygrysica, if you talk about a female tiger.)
Tygrysek is also how Tigger from Winnie the Pooh was usually called.