The UART module needs to be clocked by a source that has a deviation of +2.5% max for reliable communication. The IMO of the PSoC has a maximum tolerance of 2.5% and hence can be used for UART. But the 6MHz SLIMO clock has a tolerance of +4.2%, which is not ok for an UART. So, when SLIMO is selected as the SysClk, the DRC generates the above warning.
Refer the blog post, "UART clock accuracy requirements for PSoC1" for more details.