Dwarves currently eat and drink without being told to do so, and I see this as the same thing - maintaining a civil society. I would imagine wear autocreates jobs at a clothier like rendering fat or tanning a hide. This could be easily set in the Standing Orders menu, and can currently be mitigated by forbidding a certain amount of thread, cloth and leather if all your current stock is used up.
Letting a clothier mend clothes seems natural - if a dwarf has the clothier labor enabled, they'd give it a shot. Skill should probably determine the amount of wear that the mending actually reverts. If you asked me to mend a shirt, the tears would reopen within days. If you asked a skilled seamstress to do it instead, it might last longer than the original clothing did. On top of this, a dwarf should always seek to replace his clothing with something of equal or better quality if available.
I'd say that clothing should only be mendable up to xPig tail shirtx quality, because patches, no matter how well-done, look shoddy. Using thread alone to upgrade XShirtX to xShirtx and then using a portion of cloth for XXShirtXX to xShirtx seems reasonable.
I'd much rather have my dwarf take a short break to get her shirt fixed up by the clothier than to have her discard her shirt on the floor and end up throwing a tantrum because my cloth and leather supplies can't even keep up with the demand for children's clothes. Having the dwarf drop off her shirt would be problematic since she'd be running around topless in the meantime.
Another idea is using discarded XXQualityXX clothing as rags for patching and mending if no cloth is otherwise available, or possibly in preference to available cloth. Discarding ragged clothing altogether seems quite ahistorical, since preindustrial cloth was worth more than gold by weight due to the high labor intensity of its manufacture. Cloth was never discarded and was always reused for other purposes if possible in preindustrial times due to its value.
Since DF doesn't model most of the things rags are actually used for (dish towels, pitch torches, etc.), being able to use cloth for patches or unweaving then respinning it into thread seems reasonable. Smelters already store portions of material, so storing portions of cloth from respun rags at looms seems likewise reasonable. The quality of the material should probably be set to baseline to prevent looms from having to track stocks of every quality of cloth-portions separately.
I'm definitely +1 on the idea that higher quality clothing wears out a lot more slowly than lower quality, and REDUCING the rate of wear on clothes.
Considering dwarves seem to follow generally European pre-industrial ethics, I'd say that the shame of not being fully clothed in public should be pretty traumatic. There's a reason so many of us have nightmares about not being able to fully cover ourselves. That being said, clothing needs to be better addressed, whether through decreasing wear or increasing output.
My 200 dwarf fortresses usually require 4 or 5 full-time clothiers just to keep up with births and wear.