It's just a mortar snot holding it off line a bit... No big deal. They're replacement window, if I were you I'd pay more attention to the cold bridge - by today's standards you wouldn't close the reveal with masonary.
If you're going to replaster anyway and like your diy then consider knocking it about and fitting cavity closers.
Windows are usually installed like this to the outside.
Normally they would trim the inside and job done no problems. If you are planning to plaster them in make sure you don't bridge the DPC
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