Napoli admitted that moving to first base "felt a little strange" initially, but he's by far the Angels' best option offensively and did play there some in the minors. In fact, this could be Napoli's best chance at everyday playing time, as Scioscia has often gone with Mathis' glove over his bat when choosing his starting catcher.
Napoli has a lifetime .848 OPS, which is above average for big-league first basemen, and he's averaged 30 homers per 500 at-bats for his career. Assuming he can be somewhat passable offensively, Napoli is very capable of giving the Angels the same type of production they'd get from a midseason pickup like Paul Konerko.
Well, yes ... but whatever you gain with Napoli at first base, you lose with Jeff Mathis behind the plate. Mathis is a terrible hitter, one of the very worst hitters in the major leagues who is blessed with regular playing time. Obviously, Scioscia -- coincidentally or not, an ex-catcher -- has great affection for Mathis' defense, and given the Angels' success you can't really hold it against him. It's incredibly difficult to quantify, but perhaps Mathis' defensive contributions balance (or more than balance) Napoli's hitting. Particularly considering that "a midseason pickup like Paul Konerko" wouldn't be free.
If Napoli really can play a passable first base, this is not an unreasonable move. Especially if the Angels don't want to give up a couple of prospects or add a few million dollars to their payroll.