For the side note about MOPS - it's non-denominational and they are not supposed to push religion. I know we have a lot of non-Christian's attending who feel comfortable with the program.
As the post title states, I know this is a petty issue but it's annoying nonetheless.
I don't feel a specific pressure to dress my kids up to attend, I just prefer that they are neat and tidy whenever we leave our property. I think the biggest issue why I don't send them in grubby clothes is because everyone already has the impression that we are struggling financially, but we're not. We're young parents, we drive older vehicles, work our butts off, own a farm, do things as frugally as possible in order to have a more comfortable future. If they see us wearing old clothes, they'll be instantly offering us help in various ways and we don't want them to assume we need assistance. I know there are others out there who are a LOT worse off than we are.
So I guess if it's that annoying to me I should either just stop grumbling about treating stains

or send them in grubby clothes. Thanks for all the input
