Villian is 21/7 after 45 hands
UTG+ limps, Villian raise to $6 from HJ, I call OTB with , limper calls

pot -- $21 FLOP:
UTG+1 checks, Villian bets $16, Hero raises to $36

Is this an ok time to play back?

Personally, if I am going to play back at a CB, I hate seeing both the K and T there. Really you are looking for a few specific hands from villain...he'll fold QQ, JJ, and AQ (as long as it's not [Qd]).

To me, there are two good flops to play back at. One where it's all J and below, because any AK or AQ missed. Two where it's Axx, because you now rep the A, and he's got a tough call with KK-TT.

Flops with a K in them are generally bad to play back at, because AK gets there.

Of course read on villain is important too, but that's what I look for on a flop

Flop texture is not good to play back. There are too many possible draws, even if he didn't hit his K or T, he could elect to call your minraise with Q with a gutshot, overcard and backdoor flush draw. He could also have 89s, QJs etc, which all call your minraise.
After he calls half the deck are scare cards for you and you are not likely to be able to fire a second barrel.

So I would chose a better spot.

I tend to not bother playing back at CB's unless opponent does then excessively. SInce oppo seems tight I just give him credit for a hand and fold, especially with a 3:d party in the hand.

Heads up on a raggy flop or a flop with 1 pic card I might play back.

Why the heck are playing back with a 3 way pot? You are relying on 2 people having missed the flop and not having any kind of draw they fancy calling with, if they call and the misses the turn or the straight card misses do you fire again? Boy that could get expensive!

Plus, that flop looks like it would have hit a PFR quite nicely, and 21/7 isn't especially loose.