Having no taste; insipid; flat; as, tasteless fruit.

is the opposite of tasteful or tasty. We are talking bland, flavorless, flat, insipid, weak, dull, savorless, plain, unseasoned, unsavory, unflavored, probably unappetizing food.

Destitute of the sense of taste; or of good taste; as, a tasteless age.

not pleasing to the sense of taste

Not in accordance with good taste; as, a tasteless arrangement of drapery; a tasteless remark.

deficient in tastefulness; "coarse and tasteless luxury"

lacking flavor

lacking aesthetic or social taste