There's a lot to be said for bare walls. Busy walls actually distract from learning and also who's going to stop whatever is on the walls becoming mindless wallpaper? I think you're being a bit silly on that.
A room where disruptive or defiant students can work in silence out of general circulation is a totally reasonable facility to have. It shouldn't be a lark and having desk dividers helps students focus on their work and removes the pressure for them to further disrupt, which might result in an exclusion.
For example, I've worked with many a student with challenging behaviour. Most do actually want to do well, but they find social pressure hard to manage, especially if they feel they have to save face and keep their reputation. It's easier for them to get on in isolation and do what they need to if there's the 'excuse' of 'but that's isolation isn't it'. I've had students thank me for their results and then when I pointed out it was their effort and their work, they've said 'but you made me do it'. I always remind them I don't make them do anything. They chose to make good choices and I did what I could to help them make the choice they knew was right.
The second issue is how does isolation fit into your whole school behaviour policy. Here I think you've got more grounds to raise concerns that are reasonable and would contribute to the discussion (rather than the hysterical behaviour consultant style 'silence and plain walls is awful'). Are we looking at a school that has had repeated issues with uniform that borders on defiance so have been using isolation to send a message, or a school that's overreacting to minor things? Ive seen both. What are the consequences below isolation? How are they ran? Do SLT back staff when behaviour needs intervention above teacher level? Could a centralised lunch detention, for example, ran by SLT be an option for minor uniform issues rather than isolation so that a sense of proportion is maintained? Would SLT be willing to consider this as a way to reduce workload and sort uniform out, or are they the type of SLT who wouldn't consider that because it's too much like hard work and they'd rather under pay a member of support staff to house students for a few hours?
In terms of ongoing needs, what is the protocol for repeat offenders? How do they monitor SEND representation in isolation? If SEND students are unduely overrepresented in isolation, what will they do to evaluate existing SEND provision? For long term offenders who are persistently defiant, how does isolation fit into a support package to reintegrate? What will school be doing to aim to reduce repeat stays in isolation?
I think on some things you are being way too precious to the point of soft, but in some respects you're under-considering more pressing issues when considering an isolation policy.
But I'm prepared for the almost inevitable MN 'isolation breaks human rights and should never happen and no child can be expected to follow a simple rule... let me take a SEND case and overapply it to a student population it doesn't apply to'.