Lots of good advice already! Some additional ideas:
1) During a random moment (not while she is grumbling), tell your Mom you think the mantid longevity aspect is actually a good thing. Mantid keepers can enjoy their pets without the time committment that other popular pets demand (10-15+ years for dogs, cats, etc.)
2) As mentioned by others, if her concerns are mostly money related, she might have a different perspective if you sold some. Most moms would be shocked and impressed to hear their kid say, "Hey Mom, I was able to sell some mantids recently. Since you bought my original supplies and such, want some money to help with the bills this month?" Also, a single emergency trip to the vet for a dog or cat will often surpass the annual expenses of a mantid hobby.
3) Grin and ask if she'd prefer a son who likes bugs or a son who likes drugs.
(joking)
4) Get a copy of Orin McMonigle's book "Praying Mantids Keeping Aliens" and ask her to read the introduction page. It's a great explanation for someone who "doesn't get it."