on the face of it this looks to be a nice happy birthday sign. but let's examine the facts:

* it's stuck on a dustbin,
* it reveals the lady's age to be 50 (something she may be in denial about)
* it implies she's over-weight ('big')
* it also suggests she's the only fat old person by saying "marian is THE big 50 today".

it also serves as a daily reminder of her increased age as it's been there a few days. poor old thing.