I actually feel that scene was more of a way to show Stamets' personality than Tilly's, or at least equally so.
Stamets agreeing with her (and he having no problem with her outburst) added some depth and humanity to his character who, up to that point, was a bit uptight and stuffy. I saw that scene as a bit of a coming out for his character.
We already knew that Tilly was prone to saying something like that (i.e., lacked a filter a Starfleet officer should have); that is, it wasn't that unexpected of Tilly. However, what we we didn't expect was Stamets being just as able to display natural human astonishment about "cool" things as Tilly could.