I think the unfortunate aspect of these clones is that they're lazy. I have huge respect for anyone that innovates in payments -- I know how hard it is, and I'd imagine it's even harder to do that as part of a larger company.
What I find disappointing is when a product copies everything about Stripe -- everything from Stripe.js to the "live/test" switch to the "1/2/3" getting started pop-up when you create your account to the nomenclature of "InvoiceItems" to the ordering of tabs in the account settings.
It's certainly MasterCard's (and anyone's) prerogative to do that, but I don't think it's particularly worthy of support from other engineers and makers.
We should encourage each other any time we see great, original work -- creativity is hard, and new ideas are often delicate in the early days. It'd be really cool if MasterCard did that, but Simplify unfortunately doesn't seem to have anything that's new. (Please correct me if I'm wrong!)