As usual on a Monday, I had no idea what the theme was while solving, but looking at it now, I think it's a decent idea, with slightly light density. The UFO part always appears across word boundaries, which makes it slightly more elegant. The phrase YOUFORGOTME (23A: Annoyed "Hel-LO!") is a bit contrived, and I looked for UFOs in the last two long-ish across answers, PAGELAYOUT (18A: Web designer's concern) and CONFINEDTO (58A: Locked up in), but, well, that's the way it goes.
One possible result of the limited theme is a general decent-ness to the fill. It's not completely without its LAMER (19D: Less funny, as a joke) parts (LAUDING, AFAR, ISIN), but none of it is terrible, and you do get such gems as SCALENE (9D: Like a triangle with sides of different lengths), USURY (32D: Exorbitant interest charge), and FOGHORN (42D: Blast from a lighthouse) (Does it really come from the lighthouse, though, or does it come from a foghorn house, like at Cape Elizabeth?).
Least favorite clue today, and possibly ever: 57D: 21st word of the Pledge of Allegiance (ONE).
Not great, but a serviceable Monday.