I'm a huge fan of Kate Mulgrew. I can't speak to her talents relative to Oscar-winners, but I can say without a doubt that she's on the higher end of the spectrum of Trek actors. For example, I think she's a much better actor than Avery Brooks. She really seems to be living in whatever scene she's shooting, not just hitting her mark and reading her lines. It's the little things she does--body language, facial expressions, moving around--that just make her pop more than others.
Brooks sucked, that's fine. But I couldn't disagree with you more on Mulgrew, all those "little things she does" are exactly what annoy me. They're all so calculated and actressy, she seems to have trouble acting naturally in many of her scenes and it takes me out of the story, I feel I'm watching an actress portraying a character and not the other way around. That said, I find Mulgrew's performance in early Voyager ten times better. Unfortunately she just keeps getting worse as the show goes on...much like her character.
Also I really liked Janeway in the Cloud, I wanted the ship to feel like a family but....