Which is more acidic HOF or HOCl?
If we compare the HOF and HOCl 
1) In HOF , the oxygen atom cannot do backbonding since fluorine has no vacant orbital while in HOCl it can do hydrogen bonding it can do backbonding which reduces electron density on oxygen by a slight extent( Remember 2pπ-3pπ backbonding is not very much effective due to size difference of 2p and 3p orbitals) . But yeah this factor makes HOCl bond more polar .
But there are two more important factors-:

2[)INDUCTIVE EFFECT]-  among oxygen and fluorine , fluorine is more electronegative , so it partially pulls the sigma bonded electrons towards itself giving a  delta+ charge to oxygen .
While in HOCl , O is more electronegative than Chlorine so the opposite mechanism happens and O gets a slight negative charge.
So according to this HOF has increased bond polarity than HOCl.
3) This factor is the most important and sufficient to answer almost any acid comparison . Compare the stability of the ion formed after releasing H+ ( formally called as the conjugate base of the acid).
So the question left is simply the comparison between OF- and OCl- , 
 The first two factors again work here but another important factor is the 2p-2p electron repulsions( due to smalls ize of sigma bond the electrons in the orbitals of O and F repel each other. This factor  in itself is quiet dominating) present in OF- makes OF- less stable than OCl- 

So according to me HOCl must be more acidic than HOF.

Another important thing i wanted to draw attention to is that HOF is not the common acid that can be kept in laboratory. It is quite reactive and reacts vigorously even at 0°C , measuring its acidity is quite diffcult . I also searched online but couldn't find any data concerning Ka values for ionisation of HOF. 
I hope this helps.