Hi R.,
My honest opinion on this is that I don't think a 5 year old who's drinking out of a sippy cup and hanging out under a table is dumb. You might see it as a dumb thing (I interpret it as just having fun) and your daughter is just thinking back to her younger days and having fun with it. Maybe she was hiding under the table with the sippy cup because she wanted to avoid being noticed because of how you might react to her. (BTW- my kids used sippy cups up until this age sometimes and sometimes it's because I didn't want the spills!) Some moms worry about ..... Oh... I have to remove her crib she's 2 yrs. old, or Oh... I have to get her off the bottle because she turned one last week.... etc.... I was in no hurry and my kids did not turn out to be babies. They are mature 9 yr. olds now.
I know you are a busy mom and it's hard to make one on one time, but really try to do it weekly even if it's only for a half an hour. Maybe some cuddling while looking at her own baby pictures and videos will make her feel really good as you make sweet comments to her about the pictures/videos. Maybe a ride in the wagon, bike ride, picnic at a park .... just you and her.
I am not trying to come down hard on you by saying this..... but maybe she really isn't doing dumb things like you say she's doing. Many people over use that word and the word stupid, as well. Try thinking of other words to describe her behavior. It's difficult for young kids to articulate their feelings. (give her a couple of more years before you expect her to explain herself better.) At this age, they still need some direction with sorting out their ideas in their head.... asking Why did you.........? Can still be a tough question to explain as a 5 year old. My husband would sometimes ask questions with Why did you......? My kids would freeze and know that he's angry and would say.... "I don't know." or sometimes they would just start crying. I would jump in sometimes and say...."Well, Daddy, Christina enjoys spraying water everywhere because she thinks it's fun, but she will try harder not to get the windows wet next time and only get the flowers." It kind of bothered him that I would intervene, but I felt I needed to defend them. Now my kids are older and can answer the WHY questions on their own. (And... I'd like to think that my coaching them with their dad helped and because of the way I personally responded to their actions/behaviors.)
Good luck, R.. And I think it's great that you're asking. (I've been in your shoes with this kind of stuff so I hope my advice is useful.)
-J.