My character has just married the lovely Lydia but she continues to wear her original armour around Breezehome. I got her some tavern clothes but apparently that's not good enough. Or will she always wear armour? Looks ridiculous, and is certainly not inviting. I don't want to get into mods by the way.

She will wear whatever gear in her inventory that has the higher stats. So if you can't take out the armour, she won't wear the dress.