I think Frakes tried to add energy, character and humor to his scenes. For all the talk of how supposedly bland TNG was the actors did try to add character and energy tho their scene with little characters traits and oddities.
In my experience, whenever someone says TNG is bland, it's usually in regards to the writing (e.g. the lack of conflict between senior staff) rather than the acting.
If someone else wants to call his smile a smirk, I won't argue. He does smirk sometimes.
And sometimes he goes for the full out grin. But if anyone would like to sit down and chat by the fireside about what the smirks and the grins mean over some tea, I'll be more than happy to join in