Any adjustments that might seem out of sorts with a possible mount build?

Giant Winged Wolverine (Celestial)
Large Beast
AC: 14 (Natural)
HP: 56 (7d10+21)
Speed: 50, burrow 10, Fly 60
STR 16 (+3), DEX 14 (+2), CON 16 (+3), INT 6 (-2), WIS 12 (1), CHA 8 (-1)
Saves: CON +5
Skills: Perception +3
Senses: Keen Hearing and Smell
Passive Perception: 13
Keen Hearing and Smell: Advantage on Wisdom (Perception) checks that rely on hearing or smell.
Damage Resistances: bludgeoning, piercing, and slashing from non-magical attacks
Multiattack:
Bite: +4 to hit, reach 5 ft., (1d8 + 3) piercing
Claw: +4 to hit, reach 5 ft., (2d6 + 3) slashing

Really good walking speed and Fly speed.  Weird combo.  Plus Damage Resistances and burrow speed also and that makes it at least CR 7

BUT

Combat abilities are low, looks like CR 5 or lower (Hitpoints, damage, +4 to hit).

I see three options:

• CR 4: lower the walking speed to 20, Fly speed to 30, drop the Damage Resistance entirely (not all celestials get it)
• CR 6: change the Damage Resistance by adding 'that are not silvered', change celestial to Fey, and add 20 hitpoints.
• CR 8: add 40 hitpoints, make it the bite 2d8+3 and Claw 3d6+3, change the to hit to +7

I am trying to get it around the CR 2 or 3 for use as a Greater Steed. Lowered hit die, removed damage resistance, lowered speed to something less wonky. I was poaching the damage resist from the peryton like a power gaming jerk.

Giant Winged Wolverine (Celestial)
AC: 14 (Natural)
HP: 48 (6d10+18)
Speed: 40, burrow 10, Fly 60
STR 16 (+3), DEX 14 (+2), CON 16 (+3), INT 6 (-2), WIS 12 (1), CHA 8 (-1)
Saves: CON +5
Skills: Perception +3
Senses: Keen Hearing and Smell
Passive Perception: 13
Keen Hearing and Smell: Advantage on Wisdom (Perception) checks that rely on hearing or smell.
Multiattack:
Bite: +4 to hit, reach 5 ft., (1d8 + 3) piercing
Claw: +4 to hit, reach 5 ft., (2d6 + 3) slashing

You are focused a lot on what makes it a good mount, rather than a good creature.   Build it logical as a creature, not as a mount.  Not unless it is a monstrosity created by a Wizard to serve as mounts for his army. 

Either Flight or burrow by themselves are a +1 CR.  Together they make a mean combo.   Worse, you made it good at both walking and flight.   Makes little sense as a creature.  Yeah, the player wants it, but when would it run instead of fly?

I would start with Giant Badger.  I do not see a Dex of 14 either.  Adding Armor and hitpoints to go with it.  For me, a burrowing creature should have Darkvision.    No reason for the good Con save.  Finally I did research and a real wolverine is a cold weather adapted, so I could see adding in Resistance to Cold.  That seems like a fair celestial to exist and something that a player would still want as a mount.   If they really demand the high speeds, give up the burrow and get a Pegasi.

This would be what I think is appropriate  CR 3 version.

Giant Winged Wolverine (Celestial)  CR 3
AC: 14 (Natural)
HP: 48 (6d10+18)
Speed 20 ft., burrow 10 ft,  Fly 40
STR         DEX            CON        INT    WIS          CHA
16 (+3)    12 (+1)    16 (+3)    6 (-2)    12 (+1)    8 (-1)

Skills: Perception +3

Damage Resistance: Cold
Senses Darkvision 30 ft., passive Perception 13
Languages : It understands but can not speak Celestial and Sylvan

Keen Hearing and Smell: Advantage on Wisdom (Perception) checks that rely on hearing or smell.

Multiattack:
Bite: +4 to hit, reach 5 ft., (1d8 + 3) piercing
Claw: +4 to hit, reach 5 ft., (2d6 + 3) slashing

Yes, I thought about the resistance to cold as well. I agree with the DEX mod and movement adjustments. This is really an advancement on a current mount that the character is one level away from summoning. I was leaving off darkvision because the actual animals eyesight is pretty crappy and they are led more by hearing and smell than anything, even though they are avid burrowers.

Giant Winged Wolverine (Celestial)
Large Beast (CR 3)
AC: 14 (Natural)
HP: 48 (6d10+18)
Speed: 30, burrow 10, Fly 30
STR 16 (+3), DEX 12 (+1), CON 16 (+3), INT 6 (-2), WIS 12 (1), CHA 8 (-1)
Resistance: Cold
Skills: Perception +3
Senses: Keen Hearing and Smell
Passive Perception: 13
Languages : It understands but can not speak Celestial and Sylvan
Keen Hearing and Smell: Advantage on Wisdom (Perception) checks that rely on hearing or smell.
Multiattack:
Bite: +4 to hit, reach 5 ft., (1d8 + 3) piercing
Claw: +4 to hit, reach 5 ft., (2d6 + 3) slashing

Thank you for your assistance Mog_Dracov.