The speaker finds all of the landscape sad. Or is that because he's setting the scene for Mariana's sadness? Either way, it isn't just our main character that is full of sorrow. Every image in the poem enforces a gloomy, dreary disposition. This is just the only time he really tells us what an inanimate object actually feels.
She only said, "My life is dreary, He cometh not," she said; (9-10)
This is the first time we understand the cause of Mariana's sorrows: a man who has not returned to her. Even without the epigraph—which ties her to the character in Measure for Measure who has been jilted and so decides to live in seclusion—we can see that it's been some time since he's been around. Her sadness, then, has been brewing. Still, she seems hopeful that someday he will return.
She could not look on the sweet heaven, Either at morn or eventide. (15-16)
She's so sad that she can't even look up into the sky. "Heaven" is usually a place that people look forward to, but Tennyson is saying that she feels like she has nothing at all to look forward to. If there's anything good around her, she isn't going to notice it. Mariana is fully stuck within her mind, and can think only of her own sorrow.
without hope of change, In sleep she seem'd to walk forlorn, (29-30)
The root of her sadness is her lack of hope. Like her unwillingness to look forward to anything good, she's unable to find a reason to feel happy. The future doesn't seem to offer any hope of change, and it turns her into a zombie (not literally, hopefully). Depression can zap even the smallest amount of hope out of life, and it seems like Mariana is suffering pretty hard as a result.
but most she loathed the hour When the thick-moted sunbeam lay Athwart the chambers, and the day Was sloping toward his western bower. (77-80)
What's the time of day that Mariana hates the most? It's sunset, when the rays of light illuminate her room (which is full of dust). It also signals the end of the day, and thus the failure of the man to return. In her depression, being reminded that time is passing without any change to her mood is very difficult. It also reminds her that she's still alone, in her house, which she hasn't bothered to dust. No wonder she hates sunset.