I don't think you need to specifically mention it, but I don't think you should give a false impression either. The fact that you have been on maternity leave obviously doesn't impact on your suitability for the job, so from that point of view it is irrelevant. But in your specific circumstances it may impact on your ability to fulfil the job, if they want someone to start immediately and you are not prepared to do that because of your maternity leave.
I would suggest you don't specifically mention it, but if it naturally arises in conversation, don't conceal it either. If you are moving from a full time to a part time role they will probably ask you why, which is fair enough, and although you could make something else up as the reason, it seems a bit daft not to actually say why - there's nothing wrong or unusual in wanting to reduce hours after a baby.
Plus obviously if you do get offered the job, or get through to a second interview or any stage where start dates are discussed, you are going to have to raise it then anyway, so it would be weird if you've given a false impression in any way previously.
To clarify, it's perfectly legal for an employer to refuse a request to change from full time to part time. If the employee puts in a proper flexible working request according to the legal procedure, the employer is obliged to seriously consider that request, following the appropriate procedure and timescales, and giving full business reasons if they are unable to accommodate the request. But as long as they do that, they can refuse.