There is no instance type validation check under defaultMachinePlatform.
For example, set platform.azure.defaultMachinePlatform.type to Standard_D11_v2, which does not support PremiumIO, then create manifests:
 
# az vm list-skus --location southcentralus --size Standard_D11_v2 --query "[].capabilities[?name=='PremiumIO'].value" -otsv
False

install-config.yaml:
-------------------
platform:
  azure:
    defaultMachinePlatform:
      type: Standard_D11_v2
    baseDomainResourceGroupName: os4-common
    cloudName: AzurePublicCloud
    outboundType: Loadbalancer
    region: southcentralus

succeeded to create manifests:
$ ./openshift-install create manifests --dir ipi
INFO Credentials loaded from file "/home/fedora/.azure/osServicePrincipal.json" 
INFO Consuming Install Config from target directory 
INFO Manifests created in: ipi/manifests and ipi/openshift 

while get expected error when setting type under compute:
$ ./openshift-install create manifests --dir ipi
INFO Credentials loaded from file "/home/fedora/.azure/osServicePrincipal.json" 
ERROR failed to fetch Master Machines: failed to load asset "Install Config": failed to create install config: compute[0].platform.azure.osDisk.diskType: Invalid value: "Premium_LRS": PremiumIO not supported for instance type Standard_D11_v2

same situation for field vmNetworkingType under defaultMachinePlatform, instance type Standard_B4ms does not support Accelerated networking.
# az vm list-skus --location southcentralus --size Standard_B4ms --query "[].capabilities[?name=='AcceleratedNetworkingEnabled'].value" -otsv
False

install-config.yaml
----------------
platform:
  azure:
    defaultMachinePlatform:
      type: Standard_B4ms
      vmNetworkingType: "Accelerated" 

install still succeeds to create manifests file, should exit with error when type and vmNetworkingType setting under compute.
ERROR failed to fetch Master Machines: failed to load asset "Install Config": failed to create install config: compute[0].platform.azure.vmNetworkingType: Invalid value: "Accelerated": vm networking type is not supported for instance type Standard_B4ms 

4.14.0-0.nightly-2023-10-08-220853

always on all supported version

1. configure invalid instance type ( e.g unsupported PremiumIO) under defaultMachinePlatform in install-config.yaml
2. create manifests
3.

installer creates manifests successfully.

installer should exit with error, and have similar behavior when invalid instance type is configured under compute and controlPlane.