Hi M.. It's quite possible that she does get to thinking about things when she's working in there. And the phrase "I miss Daddy" may be code for "I'm upset about the way things are."
But nonetheless, when kids are stuck doing a job that makes them feel frustrated or overwhelmed they sometimes get the blues and start focusing on other frustrations.
At the same time, realize that for a child this age, cleaning a room does seem overwhelming. What she needs is to learn how to break this big job down into a series of little jobs. So to get the room cleaned up, without her getting into overwhelm and stalling out emotionally, go in there with her to "help".
You can take on some small token job to do. But most of your help is to assign small little tasks. "Okay, lets start with picking up all the dirty clothes, and putting in them in the hamper." "Great! Now lets get all the cuddlies and put them [wherever they go]." etc.
You may need to supervise for quite some time. Especially if she's the sort of kid who tends to make a big mess quick. (My middle child is still like that -- at 13!)
But over time, she will get the idea of HOW to clean her room.
What will help even more is if you make the proces fun. Like the room cleaning Olympics. Applaud all her progress. "You did it! Wooooo! And the crowd goes wild!" Pile on the compliments, point out the progress, and so on.
Have either of you ever seen that TV program on PBS "The Big Comfy Couch"? I think they are back on the air. I was able to introduce my 5 year old son to Lunette the clown and her "10 second tidy". We get a kick out of imitating the wacky song that plays as Lunette scurries around to toss all the toys in the toy box.
At the beginning of every clean up session, Lunette always looks around, after a morning of play, and stares with dismay as she hollers, "Hey! Who made this BIG MESS?? ...Oh. Me?" Very funny to all my kids over the years.
Of course, sometimes a child will just feel like testing you with "I'm too tired." or "I dont' feel like it. You do it." And you need to be ready to explain the consequence. The most common technique parents use is to say, "Well, we need to keep this picked up because otherwise when I try to come in your room to put your laundry away or tuck you in at night, I won't be able to get in here. So if you won't pick it up, I'll just have to put all this in a big box [or bag] and put it away in 'time out' until you're ready to take better care of it and help clean up."
Sometimes they say, "Okay." And just put it away for a couple weeks. Maybe longer if that's what it takes. The worst that can happen is she'll realize she doesn't miss all that junk and there will be less to clean up.