Oh yes...they will mat after wearing clothes....I spray a little Ice on Ice on my girls before I put any clothes on them....some people say Satin helps cut down on knots but I've found just the movement of a yorkie in clothes will cause tangles...
the longer they're in clothes...the worse the mats will form.
SO....My advise is to only wear clothes only when you go out ...or find something with satin underlining if you have this problem. Some mats are so hard you almost can't brush thru them...and it hurts the dog
The other thing I used for getting out mats is JOHNSONS NO MORE TANGLES for real babies....saturate the hair and work the knots with your fingers first - then brush
Some yorkies are lucky and don't knot up...both my girls do.