Mullins plays legitimately good CF defense. The arm is subpar but his range is upper tier and he catches whatever he can reach. For me he’s a nice 4th/5th OF to have around to play late inning defense, pinch run, and start a bit vs. RHP.
His bat is spotty, but I’ve seen worse in a plus defensive CF. Steamer and Marcel project him at .693/.694 OPS which seems like a good projection to me, though he might do a bit better if they keep him away from LHP better.
I completely reject the notion that defense is all you need at any particular position, such as SS or C. At every position, the analysis is the same: does this guy add more runs with his bat than he subtracts with his glove (or vice versa). Derek Jeter is widely regarded a poor defensive SS but he was an excellent hitter and his teams were way better off having him than not. Ditto Mike Piazza at C. Those are extreme examples but there are dozens if not hundreds of other examples. Some fans are offended by players or teams that are good with the bat but below average with the glove, but I say, whatever works. The Dodgers were a -15.3 defensive team this year, per Fangraphs. 23rd of 30 teams. Think their fans care?