Loss in a mismatched line:

When the transmission line is terminated in the resistance R = Z_o, then current and the voltage are constant all along the line, if line is perfectly lossless. The ratio of voltage to current, E/I, is equal to R and also equal to Z_o.

This is an idealized case; no line is without loss completely. Thus, as a signal goes along the line from the source to a load, the current and voltage decrease eventually. But they are always in the same ratio.

Figure--   In a matched line, E/I is constant, although E and I both decrease with increasing distance from source.