To put the obvious out of the way, you know fictional characters and fictional abilities being fictional and all not having to be consistent or make sense, although the attempts to make them so are relative in many ways and most of the time its meant to be fun and for a lot of writers and readers, but necessarily all, fun can entail just sounding possible than being possible.
As far as basic understanding as far as I know with what's believed in reality moving near and at speeds of light carries relative mass with it and energy to move that mass, or to put another way a lot of energy is required to move a something very fast and when something moves very fast its mass increases and all those things track exponentially meaning the faster the speed, the more mass, the more energy required. Some clever writer with a some understanding of physics one day even decided to implement this knowledge into Flash's power set by actually giving him the ominous sounding infinite mass punch. The problem with that and some of the prior posts is that many writers often consider infinity a number when its not, its a concept often linked or applied with numbers. If Flash reached a point where an infinite mass punch would be accurate, then not only would his footsteps long punched infinite holes through whatever medium he was using for friction (like Earth) not only would his fingers weigh more than the sums of entire galaxies, but he should probably implode taking everything we know along within him being an object of such mass in a human frame outstripping any black hole or anything we have ever known in density save for the whole big bang expansion which likewise including an incredible mass within an incredibly infinitesimal space.
Except this is comics man, and even in real life we speculate and study about special properties and matter that may move as fast as light or even faster, so its easy to cheat the above problems by saying that Flash and other characters that move that fast (and in a few instances characters like Superman, Thor, Gladiator, Silver Surfer and so on have moved or done things near or in excess of the speed of light) - by either rewriting what we know about physics, but in that case Flash's infinite mass punch wouldn't be an accurate term or description (even if his fist was the only thing carrying with it infinite mass, infinite mass is still infinite mass) but it could still allow for him to punch as hard as a writer may imagine, only explaining how the corresponding mass doesn't affect other things and why Flash can't do other stuff with the energy he is getting relative to however hard he punches might be hard, but lets be honest no writer would probably try. Or alternatively one can consider that the Speed Force grants very special properties that effectively harness an outrageous amount of power from some unknown location that is akin to a universe or dimension, so in a way infinite, and this energy not only helps Flash move extremely fast, it lets him do it without the restraint of carrying anywhere near infinite mass with him, granted the Speed Force could act as a barrier as well, shunting Flash out of any reality he may be in to some adjacent reality where the physical laws and rules and properties are different where he carries an unmeasurable always increasing amount of weight and or the Speed Force acts as a containment field that also grants Flash relative durability to allow for collisions and impacts with his body parts but greatly minimizing the normal consequences (absorption or displacement) of things with mass moving that fast being opposed by other forces.
If the Speed Force allows Flash to move at light speed without requiring him to have infinite energy to move infinite mass, then his punch force goes down dramatically and by dramatically I mean it doesn't end the universe, potentially depending on how the Speed Force exerts itself that could still mean he can destroy stars and planets potentially, since there is obviously a very very big gap between infinite mass and the mass of stars and planets. Also you'd have to consider a writers opinion on whether Flash can hit that much harder than certain other characters you can tell they consider to hit harder but can't move as fast like maybe Darkseid as an example, but maybe he moves light speed too, I am not sure.
Ultimately its up to a writer and how they understand and consider the behavior of the Speed Force or insert any other deus ex machina idea here and its affect on what's considered the reality and rules and laws of that reality wherever Flash or other as fast or faster than light characters stories occur.
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