Who are the pitchers that performing significantly worse than they have in the recent past?
Mikolas in 2022 had an 85 ERA-, 100 FIP-, 95 xFIP- (mostly just greatly overperformed his peripherals last season) compared to 101 ERA-, 101 FIP-, 101 xFIP-. He's also 34 so a slight drop off wouldn't at all be surprising.
Flaherty has struggled with injuries so hasn't pitched much the last few years but last year he had a 109 ERA-, 127 FIP-, 123 xFIP- compared to 115 ERA-, 109 FIP-, 107 xFIP- this season. Performing pretty much on par with how he did last season.
Montgomery had a 90 ERA-, 91 FIP-, 87 xFIP- last year and has a 107 ERA-, 93 FIP-, 94 xFIP-. His ERA will probably stabilize as the year goes on but peripherals are largely the same.
Wainwright is 40 and has lost has career battery mate so him having huge issues this year is not at all surprising. Especially considering he was terrible to close out last season. Has an ERA over 6 in his last 11 starts dating back to last season.
Now if you want to talk about pitchers wildly underperforming relative to past results, Burnes and Peralta are far better examples.