Does anyone know why this happens sometimes?
I'm the Karin anti-airing Ryu and then dashing. But I'm turned around so my meaty whiffs...
Hard to tell with the framerate of that .gif being bad, but it looks to me like you're going for the meaty too soon if this happens. Aside from the crouching normal moving you forward, Karin seems to be already recovering (or about to) before Ryu even lands.
Replays can be moved frame-by-frame for reference, so that'll help you in examining where it went wrong.
The downside of crossplay.
The downside of crossplay is Capcom not having the foresight of incorporating an in-game message system.
I sincerely don't have a plan against him as Mika. I just go bananas, everything he has keeps me out. He can Yoga Flame all day if he wants. I end up eating hits specifically to move closer and get in range before he ports.
Not suggesting that's how it's done but man. I haven't figured it out yet.
Counter-poking and whiff punishing are two (of several) crucial pillars in your strategy against any half-decent Dhalsim:
These are not Mika's only options, but treat it like an exaggerated form of footsies. Playing the grounded game is also important against a Dhalsim with a knack for anti-airing, especially as a character oriented around command grabs, because his ability to control the air space (be it on the ground or via V-Skill) is potent in Street Fighter V. Jumping willy-nilly does nothing but remove options on how to approach him, so do it in a timely manner as a mix-up once you conditioned him during the neutral game. Patiently walking forward, as well as timely dashes when he throws a non-EX fireball, will suffice in working him towards the corner without expending your own meter where his options (universal of a rule this may be) are severely limited. Practice jabbing his instant-air teleports on-reaction, because doing so successfully means he puts himself in a 50/50 without any prior set-ups. Especially now that even reversal teleports can be grabbed during its start-up frames. Other normals are also plenty fast to achieve the same result, but jab is the safest until you're more comfortable with this. Slides and drills are mad unsafe without near-perfect spacing, sometimes even on-hit, so be mindful of this glaring vulnerability.
Lastly, multiple characters can do the following so examine if Mika's is fast enough for this too:
If you still need more information about Dhalsim,
I've written an extensive post over in the Newbie thread detailing his strengths and weaknesses to help clear up any confusion.
Really helpful for getting out of the corner.
