Uglier means less pleasant to look at. Uglier than the first, gets uglier, uglier still.
It is the comparative of ugly, and the y turns to i as it does in happier and prettier. The word also describes situations turning nasty — the argument got uglier — which is the commoner use in reporting. Note ugliest; and that ugly comes from an Old Norse word meaning frightful.
1adjectiveuglierLess pleasant to look at.
“The new shopping centre is even uglier than the old car park it replaced.”
uglier is a comparison of appearance.
Old Norse
uggligr — fearful, dreadful, horrible in appearance
Middle English
ugly
English
ugly
English, today
uglier
Built from
ugly Middle English ugly, uggely, uglike, borrowed from Old Norse uggligr (“fearful, dreadful, horrible in appearance”), from uggr (“fear, apprehension, dread”) (possibly related to agg (“strife, hate”)), equivalent to ug + -ly. Cognate with Scots ugly, uglie, Icelandic ugglegur. Meaning softened to "very unpleasant to look at" around the late 14th century, and sense of "morally offensive" attested from around 1300. For the meaning development compare грозен (< grozьnъ), стра́шный (< strašьnъ < straxъ); foedus (< bʰeyh₂-).
Etymology from Wiktionary, CC BY-SA