Denise Crosby is just a plain bad actress. This just isn't a comment on her work on Star Trek: The Next Generation, this is a comment about her work in general.
It seems Roddenberry cast her more for her body (guy has to have priorities) than any actual skill she had as an actress. YMMV.
To quote George Bluth from Arrested Development, "Pop-pop horny".
Thank you! Denise Crosby really does my head in. The bottom line is that she bailed out of TNG after one season, then when her movie career went nowhere she tried to get back into TNG. She didnt even give it a chance, I wouldnt have minded Patrick Stewart quitting after the third season, because at least he would have given it a good try. I remember when I watched 'Trekkies', everytime the camera showed peoples art of Star Trek characters, there would always be one of Yar, even though shes one of the least liked trek characters of all time. Then when someone mentioned 'Yesterday's Enterprise', she immediately rushed out 'yeah I had to quit Star Trek to get the best episode'. The way she said best episode seemed to suggest that she was behind it all.
Her appearance in that episode was ok, but I was frustrated by the others, it just felt like a really forced twist to have her keep popping up. On Memory Alpha it says:
"Sela was devised and played by
Denise Crosby. In devising the role, Crosby stated, "
the part of Sela sort of came about from me sitting around in my house one day thinking about how much fun it was for me to go back and do "Yesterday's Enterprise", and it was so much fun that I thought what else can I do? I thought it was pretty well established that Lt. Yar and Lt. Castillo on "Yesterday's Enterprise" had something going so perhaps they had a child or Yar was pregnant when she went back into the past to fight her final battle. And I sort of thought it out and it seemed to really make sense, and there were no flaws."
Im not really a cynic, but I think its a lot more probable that Crosby was just sitting round the house thinking 'How can I get back on Star Trek?' Similiarly for 'Nemesis':
'Reportedly, Denise Crosby wanted to appear in this film as her recurring TNG character Sela, but was told by Rick Berman that it would not fit with the story'
A rare moment of clarity for Rick Berman. Again, this was not Crosby thinking of how she could improve the story, this was just her trying to get back into TNG. Unprofessional to leave so soon, unprofessional the keep trying to come back. BAH!