Simulated bode plot in Simplis is different from the result of Mathcad
The control-to-outout transfer function of the CCM buck operated in voltage-mode control is the following:

Several comments regarding your results:

in Mathcad, you have missed the pulse-width modulator (PWM) gain which is \$G_{PWM}=\frac{1}{V_p}\$ with \$V_p\$ the peak of the modulation ramp. This is your source \$V_3\$ in the SIMPLIS file. It must obviously appear in the control-to-output transfer function that will use for analytical results.
the quality factor includes many contributors as you can see. In the formulas, these are values of passive elements such as ohmic losses of the capacitor and inductor. In reality, other losses affect \$Q\$ such as the MOSFET \$r_{DS(on)}\$, the diode recovery losses and even magnetic losses. This is the reason why you may see differences between your Mathcad analysis and the SIMPLIS one or a bench prototype. To make sure you have very close results between simulations and Mathcad, chose perfect switches in SIMPLIS (and not a MOSFET), 0-\$V_f\$ diodes etc. Finally, you may be interested to import SIMPLIS simulation results directly in Mathcad to superimpose magnitude and phase curves. See here for a tutorial.

Finally, you can have a look at the 60+ simulation templates I posted here for my next book on small-signal modeling of switching converters (TOC is here). Most of the examples work on Elements, the demo version and you can explore many structures.