I think a lot of it is due to preconceptions and first impressions.
Wooden: T'Pol doesn't have Spock's sense of humour, so there's less eyebrow waggling. There's also less of the warm emotional moments of the TOS movies, though of course by that time those characters had known each other a lot longer.
Emotional: There are episodes where T'Pol is definitely required to emote: eps The Seventh, Fusion, and Impulse, plus her space-drugs arc. These may have created a negative impression, particularly as they tended to show T'Pol behaving emotionally, but only provided the explanation later (many episodes later, in the case of the space-drugs). At other times she was quite subtle in her emotions: I'm thinking in particular of the season 4 eps Home, Terra Prime, and TATV