Plain, homely, simple, unpretentious are comparable when they mean devoid of whatever embellishes or makes for superficial beauty.
Plain stresses lack of anything (as ornamentation, complexity, extraneous matter, or strongly marked characteristics) likely to attract attention.
Additionally it may suggest elegance or frugality or, with reference to personal appearance, lack of positive characteristics, and then contrasts with beautiful but without implying positive ugliness.
In reference to houses, furniture, food, and other elements of domesticity, homely sometimes suggests homey and may indicate comfortable informality without ostentation. It may connote warmth and simplicity.
With reference to appearance homely in American but not usually in British usage often falls between plain and ugly.
Simple may occasionally differ slightly from plain in implying choice rather than compulsive circumstance.
Unpretentious, stressing lack of vanity or affectation, may praise a person but depreciate a possession.