I'm on my work laptop, so the security on it won't let me pull up social media photos, but the Waterboy theme was a previous Halloween. She's dressed up as her character from Sinners this season. On my phone, she looks no less fit and no more pregnant in the current photos than she did in the Vicky Vallencourt costume.
https://heavy.com/sports/nfl/buffalo-bills/josh-allen-wife-hailee-steinfeld-turns-heads-halloween-costume/