Depends on the clothing - if you're selling 6 packs of tube socks, you wont do well in B&M - too much competition, doesnt stand out, margins are low. If you're selling lifestyle clothing, or women's high end clothing - you'll do a lot better.
The important thing is what types of clothing: hats, belts, socks, underwear, ties, etc. you can buy all that shit online. Sizing for that is fairly universal and if its slightly off then its no big deal.
Shirts, pants, tops, etc. - need to be tried on even if you know your size. I cant tell you how many times I bought a medium t-shirt from one company to find out it fits me perfectly, and then buy medium from another company to find out its too small. And returning that shit online is a colossal pain in the ass.
One thing that IS doing extremely well online these days (and this an exception to my argument) is men's suits. Dudes will just buy suits online because its cheaper and then take it to their tailor to get them fitted, because they'd have to do that regardless. Companies that have their own brand, story and product sourcing, are selling nice suits with high quality wool for under $200 out the door.