matlab – Conversion of time domain resposne to frequency domain

I would like to convert time-domain wave elevation (sea waves) for irregular waves to the frequency domain to get Jonswap Spectrum. I got the time-domain response from open software by giving the appropriate parameters. After using FFT commands and smoothening function in MATLAB I got the spectrum which is attached below. I should be getting a very smooth curve instead of fluctuations in the plot. I’ll be grateful if you could let me know the mistake or missing things in my code attached below.

clear;
clc;
Wave1= xlsread('WaveElev_Time_LC1.xlsx');
Wave=Wave1(:,2);
figure();
plot(Wave1(:,1),Wave);
title("Wave elevation in Time Domain");
xlabel('Time(s)','linewidth',3);
ylabel('Wave Elevation(m)','linewidth',3);
xlim([0 3600]);

Ns=length(Wave);
fs=1/0.0125;

fft_Wave=fft(Wave);
fft_Wave = fft_Wave(1:Ns/2+1);


psdx = (1/(fs*Ns)) * abs(fft_Wave).^2;
psdx(2:end-1) = 2*psdx(2:end-1);
freq = 0:fs/Ns:fs/2;

y = smooth(freq, psdx);
figure();
plot(freq,y,'linewidth',2);