At least in Leeta's case, you could make the excuse that it was part of her job to look sexy.
Troi's outfits were cute, but I agree, they were also absurdly gratuitous and served no practical function. At least this was eventually acknowledged in "Chain of Command"
with Jellico's very funny, but also very natural line about one of them.
But let's not confuse aesthetic qualities with acting ability. What does the sound of Nana Visitor's voice have to do with her acting? I couldn't stand Odo's voice, but that doesn't mean I thought the actor playing him sucked at acting. I think he made the wrong choice of a voice for that character, but he acted well in spite of it.