Mitchell had 32 catches and Hogan had 38. Hogan has 5 years of NFL experience and Mitchell was a rookie.
OTOH, both have had exactly one season with Brady in the Pats' system.
And they played different roles in 2016. Hogan's average depth of target was significantly greater than Mitchell's (Mitchel was actually about even with Edelman).
Given how radically different the Pats offensive game plan can be from week-to-week in terms of what receiving roles get targeted (RBs, not including Develin, saw 17 targets in the first game against the Jets last season), I'm not sure that ranking WRs individually as #1, #2, #3 is all that meaningful.
Edelman was "#1" in terms of targets, catches and yards in 2016, but White was #2 and Bennett was #3 (Bennett also led all pass-catchers in TDs and White was second). Those three pass-catchers saw 58% of all pass targets in 2016, while Hogan, Mitchell, Amendola, Gronk and Lewis split another 36% five ways. Technically, that made Hogan the "#2 WR" and Mitchell the "#3" since they were 4th & 5th on the roster in target/catches.
These numbers are bound to change in 2017, especially if Gronk can manage to stay healthy for the majority of the season. But then, the Pats have also replaced Bennett with Allen (a lesser receiving threat) and added another RB who's a demonstrated receiving threat (Burkhead).
Plus, there's Cooks to account for.
It's possible that Mitchell's role changes from 2016 to 2017. He
could begin getting more deep targets. OTOH, they already have Hogan performing remarkably well in that role. His 65.5% catch rate at his average target depth is elite by NFL standards and, I'm sure, a surprise to the Bills since they apparently didn't use him at that depth very often.
But, also, Cooks' average target depth in New Orleans comes in between what Hogan's and Mitchell's were last season.
IOW, how many targets Mitchell gets in 2017 - and it seems highly doubtful to me that either he or Hogan ascends into the top three - is way more dependent on what role the Pats need him to play against each specific opponent, and is not at all a reflection of his "talent".
It seems likely to me (depending on the health of several others) that Hogan and Mitchell come in at #4 & #5 (or even #5 & #6) in terms of targets/catches/yards in 2017, although it's possible that Mitchell ends the season with a few more targets than Hogan, depending on the circumstances that evolve in each game.
Perhaps that
would make Mitchell, technically, the "#3WR behind Edelman and Cooks. I'm just not certain that it matters all that much.