Well, it looks ridiculous, this is true, but kids like ridiculous so I don't think the fact it is silly is a problem. Kids love to dress up.
I wouldn't, however, spend £40 on ANY one outfit for a young child. It's a lot to spend on an item of clothing for someone who will grow out of it quickly and before doing so, will probably ruin it anyway.
It looks fairly simple. You could easily get an old pair of dungarees and turn them into a mouse costume.