It's not bad, but could be rainy and cold, or could be 80+. So Cal has been weird the last few years with weather, as it's harder to predict now in the Winter months. It used to be 70s all winter. Lately, the last few years, it's been rain on and off and a bit cold, for California standards.
As far as the parks go, crowds die down a bit after the Holiday season, so it's perfect to come out for 2-3 days to see and do everything.
However, do note, Jan-Mar are considered DL's off-season. So you will run into attractions that are closed for their yearly refurbs, so it's best to do as much planning and research as possible. Refurb calendars do change from time to time, so keep that in mind. Expect fireworks, Fantasmic and a few other outdoor shows to be down during these months. They try their best, but on average, these shows maybe go once a week, or are shut down altogether till later in the Spring.
But overall, it's a good time to come out and not feel the bustling crowds around you at all times - except DLR adds a ton of passholders during Christmas, so you'll always see an influx of locals that stream through the park on a daily basis.
It's a different era of DLR, as I remember as a kid, the park would close at 6pm during the weekdays and there was like nobody walking around in Jan months, ha... the days...