No. Every country has a right to either join/ratify UNCLOS or not. Joining it is entirely is voluntary.
The US has refused to join. China joined. Both had the right to make their respective choice.
Here's the thing, UNCLOS is a legally binding convention and law. Since China has joined, it is binded to UNCLOS law. If China ignore UNCLOS, then they are breaking an international law that they themselve have ratified. The bottom line is, China has a legal obligation to obey UNCLOS regardless of whether the US join or not. However, If the US ignore UNCLOS, the US is not breaking any law because it has never ratified UNCLOS.
So you cannot compare China to the US in terms of the UNCLOS law.
As for the common suggestion that the US is a hypocrite for not joining UNCLOS, or that they must not have any say in the dispute until they join/ratify UNCLOS, these suggestions has bad reasoning.
Suppose both of you two have voluntarily entered into a legally binding business partnership contract and then both of you later on have a contractual dispute.
Suppose I'm a lawyer being hired by one of you to participate in your legal dispute. I, as a non-signatory third party to your business partnership, can still make comments and have a say in your legal dispute. Eventhough I have never myself joined your legal business partnership contract, I can still participate in your dispute resolution/mediation process because I have been asked(hired) by one of the dispute party to participate in it. For example I, as a lawyer, can argue on behalf of the plaintiff that the defendant has broken its contractual obligations, argue for certain ruling against the defendant, etc.
So it is the same thing for the SCS dispute. The US is not signatory to UNCLOS while both the Philippines and China are signatories. But it doesn't mean that the US cannot have any say in their UNCLOS dispute case. If the Philippines welcome the US to do so, then the US can.