Does the Server Key Exchange sign a hash or text
Signature generation includes hashing. So there is no need to perform hashing separately. This is why the signature schemes that are used will always indicate the hash to be used - either directly or indirectly using parameters.

Without the hash the signature would be vulnerable to key-only existential forgery attacks. I don't see how that would influence the security of TLS, but TLS in general tries to use standardized primitives within the protocol 

Note that TLS up to 1.2 contains rather a lot of legacy constructions that would be considered non-standard by now, TLS 1.3 tries to resolve this by using RSA-PSS and SHA-2 (in the 256, 384 and 512 bit versions) which is provably secure if RSA and SHA-2 are secure.

So yes, hashing is used, but only as part of signature generation.