Can I mix up plural with singular to resolve pronoun ambiguity?

Technically, this is a dangling modifier. It has nothing to do with plural or singular. Dangling modifiers feel awkward because they do not attach to anything. Most people will understand you, but in a pinch, you should rewrite it.

Right now, the sentence has "Computers" as subject "larger screen" as object. "The smartphones" is modifying "the screen." "The reason" is the dangling modifier.

I think what you were trying to do is have "Computers be necessary" and everything else modifying that idea. That would be more like:

Since smartphone screens are small, Computers are still necessary.

Something like that. But not precisely sure if this is your context. Hope that helps.