On average, yes, foals tend to be weaker than their parents, because for ratings it's not a pure inheritance from the parents, there's a built in random factor. (citation needed, but according to HEE legend, Eve said so a long time ago in a forum post far far away, written on a stone tablet that has since been captured by a band of marauders riding Adam's Auction horses...) So that's not a major concern if the foal is lower rated by one rating. You should view ratings like you view report cards back in school: An "A" typically meant a score of 90 to 100, where a "B" is 80 to 89. So she might be a "P" while closer to the "E" end.

Also, her personal rating is not a perfect reflection of her breeding potential. On my breeding account, I've had PPP mares consistently produce EEPs, while other EEEs would produce PPPs and even down to Superbs. The only real way to know is to try her out, more than once... Don't be like some people who breed a WWW just to geld and free range because the very first foal was a PPP (Yes, some have done that. True story. You can read about it on some of the ancient cave wall writtings)

Just to throw in a little statistic as to what to expect, I do pasture breeding on Testube Laboratories where almost all the stallions are EEEs and the mares are at least PPPs and most have one or two Es. Out of 756 foals this year, 406 are PPP+, so 350 have one S or lower. 238 are exactly PPP, which leaves 168 with one or two Elites and only 2 are EEEs.