Unattractive icons of Bluetooth Manager, Gigolo, Network Wifi, etc.

Because the desktop files use the 16px icons of actions, mimetypes, panel and places and not the icons of apps and categories. Therefore, you need to change manually the icon names in the desktop files to catch the icons of apps. You can pick of categoric and generic icons:

accessibility	computer	login	scissors
addressbook	database	mail	screenshot
agenda	debian	maps	search
application	desktop	mate	settings
applications-engineering	dictionary	menu	share
applications-interfacedesign	document	microphone	shield
archiver	download	monitor	shop
audio	ebook	mouse	star
bluetooth	effects	music	steam
book	emacs	network	sublime-text
bookworm	file-manager	night	subtitleeditor
broom	film	notepad	tablet
browser	flash	notification	tag
bug	font	package	television
caffeine	gameboy	paint	terminal
calculator	git	panel	tool
calendar	hammer	partitions	update
camera	harddisk	password	upload
cdrom	help	phone	usb
charmap	icons	plasma	user
chat	image-viewer	plugin	video-alt
chess	image	power	video-card
cipher	java	ppa	voice
clipboard	keyboard	printer	wallpaper
clock	kronometer	programming	weather
coin	language	qt	webcam
color-picker	leaf	radio	wifi
colour	library	ruby	workspace