Fuck everything about your wife's mom, more than Lent, really.

I don't think there's anything wrong, and potentially a great deal of good, that can come from giving up an enjoyable habit or an enjoyable-but-negative behavior for a predetermined, finite period of time. For those with really bad habits it allows them to see a "light at the end of the tunnel" where it's easier to quit in the short-term because they know they can go back to it in the long-term. Sometimes, people will choose to stay quit after they get 30 or 40 (in the case of Lent) days of abstinence under their belt. I don't think there's even anything that wrong about mildly complaining about how you miss what you've given up, for a while.

It's not your wife's fault when her birthday is and it's not like her mother is oblivious to that date either. Is your wife not supposed to celebrate her birthday because her mother's giving something up? (Well excuuuuuuuse me, Ms. klein's-mother-in-law.) It's not even like there won't be plenty of cakes for MIL to eat after the end of Lent.

If I choose to make a lifestyle change, small or large, temporary or permanent, I can't expect everyone else to accommodate me by hiding my poor delicate feelings from other behaviors. Not even my family.

Your MIL is silly. At best.