I’m no expert, but I can tell you what worked for me when I was playing Mika.
Stop using stomp chop as you bnb. Its an easy combo because its a target combo but its very punishable on block (-6) and you’re probably also leaving a lot of damage on the table by using it when you get openings. You’ll still be using stomp chop here and there (its a great, confirmable follow up to charged st.HK for instance) but you don’t want to build the habit of using it a lot.
Practice st.MP -> st. HP xx H/EX Peach as your bnb. It isn’t a particularly difficult link (especially in the context of SFV) and it nets you good damage. You can use that for punishes in a lot of situations as well (I found Mika’s CCs to be kind of awkward so I usually stuck with that combo). st.MP is also fast enough to fairly easily link from her cross up (j.LK) so you can use that combo from any jump-in situation. As you get more practice you can upgrade that combo to st.MP -> st.HP xx super for bigger damage when the opportunity arises.
Practice using st. MP as a poke. I found it awkward to use while trying to stay mobile because I would get intermittent claps instead of the regular st.MP but keep working on it. In my experience st.MP was Mika’s best poke: fast, decent range, can convert to multiple different combos on hit, and while it is negative it isn’t punishable. The hardest part is not accidentally getting claps when you don’t want them (which are unsafe enough that even Bronze players will punish you for them sometimes).
Also practice linking EX Peach from st.MP. Not cancelling, actually linking it like you would the st.HP. Its not that hard to do (st.MP is very + on hit which gives you a decent window to work with combined with the inherent system leniency with links) and its a great confirm to have in your pocket.
Learn one good, consistent V-Trigger combo and one V-Trigger set up from block. Mika’s V-Trigger is one of her best features and it is surprisingly easy to convert those two little red bars into a bunch of free damage, or at least good positioning. cr.HP is normally a dangerous move to throw around but with V-Trigger stocked it leads to either solid damage on hit or a set up into damage on block. That second part I never got good at, but I’ve seen higher level Mika players really put their opponent’s in a pinch in those situations.
cr.MP is your go to anti-air. Drill that as much as you can so it becomes habit when you see jumpers. However you also need to recognize when it isn’t going to hit and react accordingly. Because Mika shuffles forward just a little bit, it can be hard to use against close jump-ins (and is worthless against cross ups). Cross ups in general are things that you need to either grin and bear (block) or work harder to not let your opponent get into that situation. One thing that can be helpful against cross up attempts is that since you’re naturally going to go for a crouching normal against a jump in with Mika, you can use cr.HP to roll under the cross up attempt. It gets you a little bit of breathing room and avoids the situation entirely, but don’t get too predictable with it.
Watch as many Fuudo matches as you can get your hands on. He is one of the most consistent Mika players and I think his style of using the character is the way to get the most long term success.
Good luck with Mika! She’s a really fun character to use when she works. I found that the hardest part was actually getting in and getting momentum started especially when dealing with patient players and/or against characters that can control space well (relative to how well anyone controls space in SFV I guess). For that reason, you need to have really drilled your bnbs, your confirms, and your set ups so you can maximize your payoffs when you get in there.