There are plenty of music-making apps on iOS or Android. However, many of them are designed primarily for iPad/tablet, so trying to use them on a phone feels cramped and unpleasant (tiny text, tiny buttons, lots of pinch zooming to interact with anything).
The only exception I can think of is Koala, which has big and clear layouts that work great on phones. Are there any others I’ve missed?
pure acid
elastic drums
patterning
Hammerhead
Tweakybeat
Nanoloop
iElectribe
Frum (not sure if it’s still about/works)
Grooverider
Caustic
Edit I also really love Mirack on the phone, might stray towards the things you are trying to avoid but the brutal power is worth the pinching etc 
me too actually. A bit fiddly 
I have killed time on a delayed subway train by playing with Hammerhead and Mononoke. Some of the controls will be fiddly. Probably the same is true of Pure Acid. iKaossilator is definitely usable on an iPhone but it is hardly a serious app. If your intention is anything more than killing time, it’s hard to think of much (though I admit I have been purchasing for iPad, it’s just that some such apps also show up on my iPhone).
Ableton Note was designed primarily for the phone.
SynprezFM is a great FM synth with lots of presets available. The keyboard is a little snug on a phone but you can resize it and you can also use a USB MIDI keyboard.
I also downloaded this app called Sur Sadhak which has premade patterns with tabla and tanpura. It’ll keep playing in the background if you load another app, so you can get a beat going and jam over it in a different app.
QiBrd and Saucillator are also pretty fun for XY pad driven synth sounds.
Apps that work well on iPhone screens:
Production: Loopy Pro, Garageband, AUM, Audiobus, 4pockets Progressions, Tonality
Synths: Spacecraft, Moog Model 15, Zeeon, Animoog Z, Geoshred, Roli Noise, Tardigrain
Obviously, bigger is better. Nevertheless, some great apps manage to be bad on both form factors: e.g. Korg iWavestation/Module.
Doesn’t look like anyone had mentioned Drambo?! It’s like an Octatrack with eurorack modules in your phone. The ui is packed but very easy to get around.
Mirack
Definitely my go to app
I bought Fluss, Hainbach’s granular instrument made with Bram Bos, to use on my iPad, but it works nicely in iPhone too
Drambo works great on an iPhone. Nanoloop isn’t bad either.
I’ve made hundreds of beats on beatmaker 2 for iPad. Such a great app. Just simple enough.
I forgot about Fluss! The user interface has some clever adjustment features, such as showing you half at a time on a small screen, with the other half just a touch away. It also doesn’t have a lot of controls.
Looks like Jamm Pro is available for Android now. Anyone dig into it while it was on the IPHONE?
Pegboard is fun on phone
well, what else fits my iPhone SE screen (which is the smallest iPhone):
Sunvox has scalable UI so you can adjust the “zoom” of text, buttons and DPI. Works well on small screens since Palm OS days.
+1 on Drambo. The design translates surprisingly well to the iPhone. Also good on an iPad mini.